complete/fish: improve shell completions for fish

- Stop using `-n __fish_use_subcommand`. This had the effect of
ignoring options if a positional argument has already been given, but
that's not how ripgrep works.

- Only suggest negation options if the option they're negating is
passed (e.g., only complete `--no-pcre2` if `--pcre2` is present). The
zsh completions already do this.

- Take into account whether an option takes an argument. If an option
is not a switch then it won't suggest further options until the
argument is given, e.g. `-C<tab>` won't suggest options but `-i<tab>`
will.

- Suggest correct arguments for options. We already completed a fixed
set of choices where available, but now we go further:

  - Filenames are only suggested for options that take filenames.

  - `--pre` and `--hostname-bin` suggest binaries from `$PATH`.

  - `-t`/`--type`/&c use `--type-list` for suggestions, like in zsh,
  with a preview of the glob patterns.

  - `--encoding` uses a hardcoded list extracted from the zsh
  completions. This has been refactored into a separate file, and the
  range globs (`{1..5}`) replaced by comma globs (`{1,2,3,4,5}`) since
  those work in both shells. I verified that this produces the same
  list as before in zsh, and the same list in fish (albeit in a
  different order).

PR #2684

crates/core/flags/complete/encodings.sh

@@ -0,0 +1,29 @@
+# This is impossible to read, but these encodings rarely if ever change, so
+# it probably does not matter. They are derived from the list given here:
+# https://encoding.spec.whatwg.org/#concept-encoding-get
+#
+# The globbing here works in both fish and zsh (though they expand it in
+# different orders). It may work in other shells too.
+
+{{,us-}ascii,arabic,chinese,cyrillic,greek{,8},hebrew,korean}
+logical visual mac {,cs}macintosh x-mac-{cyrillic,roman,ukrainian}
+866 ibm{819,866} csibm866
+big5{,-hkscs} {cn-,cs}big5 x-x-big5
+cp{819,866,125{0,1,2,3,4,5,6,7,8}} x-cp125{0,1,2,3,4,5,6,7,8}
+csiso2022{jp,kr} csiso8859{6,8}{e,i}
+csisolatin{1,2,3,4,5,6,9} csisolatin{arabic,cyrillic,greek,hebrew}
+ecma-{114,118} asmo-708 elot_928 sun_eu_greek
+euc-{jp,kr} x-euc-jp cseuckr cseucpkdfmtjapanese
+{,x-}gbk csiso58gb231280 gb18030 {,cs}gb2312 gb_2312{,-80} hz-gb-2312
+iso-2022-{cn,cn-ext,jp,kr}
+iso8859{,-}{1,2,3,4,5,6,7,8,9,10,11,13,14,15}
+iso-8859-{1,2,3,4,5,6,7,8,9,10,11,{6,8}-{e,i},13,14,15,16} iso_8859-{1,2,3,4,5,6,7,8,9,15}
+iso_8859-{1,2,6,7}:1987 iso_8859-{3,4,5,8}:1988 iso_8859-9:1989
+iso-ir-{58,100,101,109,110,126,127,138,144,148,149,157}
+koi{,8,8-r,8-ru,8-u,8_r} cskoi8r
+ks_c_5601-{1987,1989} ksc{,_}5691 csksc56011987
+latin{1,2,3,4,5,6} l{1,2,3,4,5,6,9}
+shift{-,_}jis csshiftjis {,x-}sjis ms_kanji ms932
+utf{,-}8 utf-16{,be,le} unicode-1-1-utf-8
+windows-{31j,874,949,125{0,1,2,3,4,5,6,7,8}} dos-874 tis-620 ansi_x3.4-1968
+x-user-defined auto none