fix: add zsh autocomplete setup and file permissions instructions to completion:install

[benhancock]

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Summary

Fixes Completions don't work

There are at least two issues that might cause completions to fail after running completion:install:

1. On Linux/MacOS, the completion script may not have the necessary executable permissions.

• Fix: Run chmod +x with the file path to the completion script

2. Netlify uses Tabtab for completions, and with zsh, Tabtab (and other) completions do not work until compinit has been run.

• Fix: Add the line autoload -U compinit; compinit to the user’s ~/.zshrc file above the Tabtab config line to load and then run compinit

This PR addresses both of these issues by:

1. automatically outputting the chmod +x command with the appropriate file path for the user to copy and run if needed after the installation is complete;
2. prompting the user to have the autoload -U compinit; compinit line added to the top of their .zshrc file automatically if not already present.

Here's what the netlify completion:install output looks like before:

Here's what the netlify completion:install output looks like after our changes:

This PR also creates a new test file for completion:install with tests for the compinit -> ~/.zshrc functionality.

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[dylanspyer]

We decided not to modify file permissions on the user's behalf and instead output the chmod command for the user to run manually. This ensures that the user maintains ownership of their security and configuration settings.

[DanielSLew]

Looks great! Just a few changes that I've requested