[Bash completion]: Respects .file() extensions, uses _filedir

[gwynne]

This PR implements two TODOs left over in the Bash completion script generator code:

1. CompletionKind.file(extensions:) did not actually honor the extensions array. It now does so.
2. CompletionKind.file(extensions:) and .directory now both preferentially invoke the _filedir shell function from bash-completion (v1 for Bash 3.2 & v2 for Bash 4.2+), if it is available in the shell environment at the time, falling back on compgen if it is not.

Note: I could not find any open issues referring to these concerns.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[rauhul]

@swift-ci please test

[gwynne]

@rauhul I have to be honest; this is my first time tinkering with Bash completion 😅. Here are the references I used in coming up with this:

• The use of compgen -X !*.ext to handle extensions: _comp_compgen_filedir() guts on tip of trunk bash-completion 2.x - it seems that the maintainer has planned a truly massive overhaul for the "2.12" release.
• Making sure Bash 3.2 (for non-Homebrewed macOS bash users) works: The original _filedir from bash-completion 1.x.
• Also for extensions handling: A use example in the underlying completion generator used by yq
• Directory handling: The extra CDPATH handling built into bash-completion
• Use for files without extension matching: The builtin minimal fallback handler

[rauhul]

@swift-ci please test

[rauhul]

@gwynne it looks like some tests need to be updated:

             if declare -F _filedir >/dev/null; then
               _filedir 'txt'
               _filedir 'md'
               _filedir 'TXT'
               _filedir 'MD'
+              _filedir -d
             else

[gwynne]

@gwynne it looks like some tests need to be updated:

Whoops, sorry! Incoming momentarily 😅

[rauhul]

@swift-ci please test