Provide shell auto-completion support for fish

[brettcannon]

E.g. get the fish shell to be able to provide auto-completions.

Some shells might get this for free via #24.

[brettcannon]

Might also want to somehow provide completions for Python itself.

[brettcannon]

Looks like completions in fish should be doable.

The complete command lets you register potential completions. This can bare minimum cover the static options (e.g. --help). To handle versions, that requires knowing what the currently supported versions are since there are an unbounded number of options. This will require either figuring out how to use something like awk to parse --list or an alternative output to make it easier to parse.

Potential use of awk for parsing is:

py --list | awk '/│[[:blank:]]+([[:digit:]]+\.[[:digit:]]+)[[:blank:]]+┆[[:blank:]]+.+[[:blank:]]+│/ { print $2 "|" $4 }'

It looks like read could be used to insert parts of a line into environment variables. The read docs show how then a while loop could be used to parse something line-by-line, otherwise xargs could be used. This should be enough to provide accurate completions with a help line that specifies what command will be executed.

The complete command has a -n option that could be used for exclusivity, but that seems overkill as fish's own completions don't bother with this.

The -w option for complete can d used to add all relevant Python options. And the built-in completions for python in fish are smart enough to cover versions, so there isn't a need to specify a specific command or binary that's version-specific.

One unknown is how to provide major-only completions compared to major-minor completions. Basically how do you get the first instance of a major version from py --list for an arbitrary set of potential major versions?

[brettcannon]

https://stackoverflow.com/questions/26867277/print-unique-lines-based-on-field suggests awk could be used to also do major-only completions. Probably should save simplified output in order to avoid multiple calls to py.

[brettcannon]

https://fishshell.com/docs/current/cmds/contains.html and https://fishshell.com/docs/current/tutorial.html#lists would also work instead of using awk to do major-only. Probably having awk print out major-only, major-minor, and executable would let iterating through once by adding the first time a major is seen and then append to the array of seen major versions.

[brettcannon]

Accepting this idea in #84 for fish.