feat: add bash, zsh, and fish completions for termux-tools

[SjnExe]

Adds tab completions for pkg, termux-open, termux-backup, termux-restore, termux-info, and chsh across Bash, Zsh, and Fish. Integrated into Autotools via completions/Makefile.am.

[SjnExe]

If someone knows how to build binary and test this, please do and give me results

[TomJo2000]

It would be nice for the pkg completions to use the existing apt or pacman completers.
Although I'm not 100% sure how to properly call another completer in a completer or if that's even possible.

I think complete -F should handle it for Bash, but I'm unsure about Zsh and Fish.

[SjnExe]

Updated completions according to the review comments:

• Subcommand aliasing: Matched case / switch patterns across Bash, Zsh, and Fish with the wildcard logic in pkg.in (add|i*, f*|rei*|un*|rem*|rm|del*, sh*|inf*), enabling completion for shorthand invocations such as pkg in, pkg rm, and pkg sh.
• Package manager detection: Sourced termux-setup-package-manager in an isolated subshell with session caching and clean fallbacks for apt/pacman across Bash, Zsh, and Fish.
• Fish completion syntax: Replaced hardcoded checks with dynamic commandline token evaluation and native Fish switch/case patterns.
• Verification: Built from source and verified end-to-end completion resolution directly in Termux across Bash, Fish, and Zsh.

[SjnExe]

@TomJo2000 Now everything addressed?

[TomJo2000]

@TomJo2000 Now everything addressed?

I've left you a comment above regarding the LLM's "response" for why it doesn't wanna do this in a sane way (#390 (comment)).

Other than that I'm okay with the changes in the latest push (1ecb24a..fd1965b).

One more thing we should probably add as part of this PR is proper highlighting for the completions files in GitHub's code view.
That can be done via the .gitattributes file.
e.g.

# File syntax
‎completions/bash/* linguist-language=bash
‎completions/fish/* linguist-language=fish
‎completions/zsh/* linguist-language=zsh

(The fish line isn't strictly necessary but I think for consistency having it in there would be a good idea.)

[SjnExe]

Added .gitattributes

[TomJo2000]

Highlighting appears to work though not within this PR.

[SjnExe]

Yeah, GitHub only applies .gitattributes to PR diffs once it's in the base branch. Good to see it picked up on the tree view.

[SjnExe]

@TomJo2000 PR is ready for merge, right?

[TomJo2000]

Most of what I've written for the Bash completion also applies for Zsh.

[SjnExe]

Addressed the review feedback in the latest push:

• Prefix substitution: Renamed chsh and pkg across Bash, Fish, and Zsh to .in templates using @TERMUX_PREFIX@, with substitution handled via $(do_subst) in completions/Makefile.am.
• Bash refactoring: Extracted _pkg_available_packages and _pkg_installed_packages helper functions to eliminate repeated if/else checks inside the subcommand case block.
• Syntax & quoting: Quoted assignments, passed $setup_pm directly as $0 to the subshell, and switched silent status checks to &>/dev/null across Bash and Zsh.
• Zsh matching: Added se*) and *) fallthrough cases to keep parity with Bash.
• Empty completions: Non-package subcommands (update, upgrade, clean, autoclean, etc.) return empty COMPREPLY=() to prevent shells from falling back to arbitrary local directory path completions.

[SjnExe]

@TomJo2000 Now everything good?

[SjnExe]

Updated chsh across Bash, Fish, and Zsh to check a list of known shells against PATH and only return installed ones.

Reverted chsh back to a regular completion file since it no longer needs @TERMUX_PREFIX@, leaving only pkg in the .in substitution list in completions/Makefile.am.

[SjnExe]

@TomJo2000 Now everything addressed? No issues?

[SjnExe]

Updated the known shells list across all three shells:

• Added ash, bsh, cicada, eltclsh, etsh, tsh, and preemptively added pwsh.
• Replaced oksh with loksh.
• Removed ion and yash, kept xonsh.

Also ran a full compilation and sandbox installation/uninstallation test via DESTDIR—all completions and xdg-open symlinks installed and uninstalled cleanly with 0 leftover files.

[SjnExe]

@TomJo2000 Now good?

[TomJo2000]

I'll need to look at it after work.
But this is an iterative process.

Might take a while to find and address everything.
I'd also like to get some other people to take a look at these.

[SjnExe]

Added beanshell alongside bsh across the completion scripts for all three shells.

[SjnExe]

Added osh and ysh (from oils-for-unix) to known_shells across Bash, Fish, and Zsh.