Add tab completion for julia +channel selection

[IanButterworth]

Add shell tab completion so users can type 'julia +' and see available channels. Completions for both juliaup commands and julia channel selection are auto-installed for bash and zsh, and available for all 6 supported shells (bash, zsh, fish, powershell, elvish, nushell).

ian@Mac julia % julia +
1.10          1.11-nightly  1.11.6        1.12-nightly  1.13          1.7           1.9           n             pr60943
1.11          1.11.5        1.12          1.12.4        1.13-nightly  1.8           lts           nightly       release

and julia +pr{tab} etc.

Changes:

• Add hidden '_list-channels' subcommand that prints sorted installed channel names, used by completion scripts
• Generate per-shell completion scripts that combine juliaup subcommand completions with julia +channel completions
• Write pre-generated completion files to ~/.juliaup/completions/ during initial setup and self-update to avoid eval overhead on shell startup
• Add 'source' line to managed shell init blocks in .bashrc/.zshrc
• Add hidden '_post-update' subcommand invoked by the old binary after extracting a new one, to refresh completion files and shell init blocks
• Update README to document auto-installed completions