Add 'files' highlighter

This new highlighter highlights names of existing files on the command line, in
the style of LS_COLORS.  It can pull its configuration directly from LS_COLORS,
or can be configured separately.

docs/highlighters/files.md

@@ -0,0 +1,49 @@
+zsh-syntax-highlighting / highlighters / files
+----------------------------------------------
+
+This is the `files` highlighter, that highlights existing files appearing on the
+command line.
+
+
+### Quickstart
+
+If you are happy with your `LS_COLORS`, simply add the following line to your
+`.zshrc` after sourcing `zsh-syntax-highlighting.zsh`:
+
+```zsh
+zsh_highlight_files_extract_ls_colors
+```
+
+
+### Configuration
+
+Files are colored according to the associative arrays `ZSH_HIGHLIGHT_FILE_TYPES`
+and `ZSH_HIGHLIGHT_FILE_PATTERNS`.  The values of `ZSH_HIGHLIGHT_FILE_TYPES` are
+color specifications as in `ZSH_HIGHLIGHT_STYLES`, and the keys define which
+file types are highlighted according to that style (following `LS_COLORS`):
+
+* `fi` - ordinary files
+* `di` - directories
+* `ln` - symbolic links
+* `pi` - pipes
+* `so` - sockets
+* `bd` - block devices
+* `cd` - character devices
+* `or` - broken symlinks
+* `ex` - executable files
+* `su` - files that have the suid bit set
+* `sg` - files that have the sgid bit set
+* `ow` - files that are world-writable
+* `tw` - files that are world-writable and sticky
+* `lp` - if set, the path-component of a filename is highlighted using this style
+
+If a file would be highlighted `fi`, then it can be highlighted according to the
+filename using `ZSH_HIGHLIGHT_FILE_PATTERNS` instead.  The keys of this
+associative array are arbitrary glob patterns; the values are color
+specifications.  For instance, if have `setopt extended_glob` and you write
+
+```zsh
+ZSH_HIGHLIGHT_FILE_PATTERNS[(#i)*.jpe#g]=red,bold
+```
+
+then the files `foo.jpg` and `bar.jPeG` will be colored red and bold.