main: Allow for "]" in shell aliases

PR #776 fixed an issue with complex aliases and expansion. However, this change
also introduced a problem with aliases which contain `]` (for example, commonly
seen on macOS: `alias ]=open`), due to using an associative array `seen_alias`,
indexed by the alias name. Due to `"$seen_alias[$arg]"`, it would fail when
`$arg` is expanded to anything containing `]`'. Thus, typing `] /` would result
in:

```
> ] /
(anon):unset:3: seen_alias[]]: invalid parameter name
```

This change fixes the issue by ensuring we properly access keys in the
associative array `seen_alias`.

Older versions of zsh have issues with map keys having special
characters, especially lacking ways to remove such keys. The
issue is described in detail in
https://unix.stackexchange.com/questions/626393/in-zsh-how-do-i-unset-an-arbitrary-associative-array-element.

This fix uses proposal from
[zsh-workers/43269](https://www.zsh.org/mla/workers/2018/msg01073.html),
discovered by Stephane Chazelas, that boils down to avoid removing keys
from the map, and reconstruct the map anew with some keys omitted.

Co-authored-by: @phy1729

highlighters/main/main-highlighter.zsh

@@ -601,16 +601,19 @@ _zsh_highlight_main_highlighter_highlight_list()
       (( in_alias[1]-- ))
       # Remove leading 0 entries
       in_alias=($in_alias[$in_alias[(i)<1->],-1])
-      (){
-        local alias_name
-        for alias_name in ${(k)seen_alias[(R)<$#in_alias->]}; do
-          unset "seen_alias[$alias_name]"
-        done
-      }
       if (( $#in_alias == 0 )); then
         seen_alias=()
         # start_pos and end_pos are of the alias (previous $arg) here
         _zsh_highlight_main_add_region_highlight $start_pos $end_pos $alias_style
+      else
+        # We can't unset keys that contain special characters (] \ and some others).
+        # More details: https://www.zsh.org/workers/43269
+        (){
+          local alias_name
+          for alias_name in ${(k)seen_alias[(R)<$#in_alias->]}; do
+            seen_alias=("${(@kv)seen_alias[(I)^$alias_name]}")
+          done
+        }
       fi
     fi
     if (( in_param )); then
@@ -890,7 +893,9 @@ _zsh_highlight_main_highlighter_highlight_list()
         (){
           local alias_name
           for alias_name in ${(k)seen_alias[(R)<$#in_alias->]}; do
-            unset "seen_alias[$alias_name]"
+            # We can't unset keys that contain special characters (] \ and some others).
+            # More details: https://www.zsh.org/workers/43269
+            seen_alias=("${(@kv)seen_alias[(I)^$alias_name]}")
           done
         }
         if [[ $arg != '|' && $arg != '|&' ]]; then

highlighters/main/test-data/alias-brackets.zsh

@@ -0,0 +1,41 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2021 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+# Have to use cat here as it must be a command that exists.
+# Otherwise, the test would fail with the first token being recognized
+# as an "unknown-token".
+alias ]=cat
+
+BUFFER='] /'
+
+expected_region_highlight=(
+  '1 1 alias' # ]
+  '3 3 path' # /
+)