other: Add autocomplete file generation

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

Adds shell completion generation as part of the build, as well as tweaking install scripts/templates/CI to use them.

Many of the changes are unashamedly based on ripgrep's setup. For future reference:

• PKGBUILD
• build.rs file
• Documentation/install method for completion

Issue

If applicable, what issue does this address?

Closes: #140

Type of change

Remove the irrelevant ones:

• Other (something else - please specify if relevant)

Test methodology

If required, please state how this was tested:

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

Checklist

If relevant, see if the following have been met:

• Change has been tested to work
• Areas your change affects have been linted using rustfmt
• Code has been self-reviewed
• Code has been tested and no new breakage is introduced unless intended
• Passes CI tests
• Documentation has been added/updated if needed (README, help menu, etc.)
• No merge conflicts arise from the change

Other information

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