Support system-wide configuration for zigup directory

[ielm]

Issue

Currently, all zig compiler installs through zigup are stored in $HOME/zig by default. I would like to configure that my compilers be stored in $HOME/.zigup/compilers. In the current version of zigup, I need to pass --install-dir $HOME/.zigup/compilers each time I fetch a compiler version. I would like to set a globally configured "install dir" that zigup respects unless explicitly passed an --install-dir argument.

Solutions

There are a couple ways to do this.

1. Environment variables

The simplest way would be to create an environment variable:

# This might store a `$ZIGUP_HOME` env var? 
zigup --set-install-dir $HOME/.zigup 

And subsequent calls to zigup might check for $ZIGUP_HOME existence before proceeding.

2. Use a configuration options file

2.1. A config file in $ZIGUP_HOME

Rustup, the zigup equivalent for rust, has a default $RUSTUP_HOME at $HOME/.rustup which can be overridden by setting the env var. The settings.toml file provides configuration options to set the default toolchain and to allow for project-specific toolchain overrides (configured through the CLI):

default_toolchain = "stable-x86_64-unknown-linux-musl"
profile = "default"
version = 12

[overrides]
"/home/username/d/example_project_using_nightly_build" = "nightly-x86_64-unknown-linux-musl"
"/home/username/d/example_project_using_specific_build" = "1.67-x86_64-unknown-linux-musl"

Zigup might follow a similar convention and have a settings.toml or settings.json file at $ZIGUP_HOME that allows for fine-grained configuration. This also addresses issue #69.

2.2 A known folders search

zls uses a configuration file called zls.json which is stored in a known folder. The known-folders project provides the set of utilities needed for this. zigup can take a similar approach and search for a zigup.json configuration file in known folders. (And for MacOS also look for $HOME/.config/zigup)

Folder	Windows	xdg	MacOS
Roaming Configuration	%APPDATA%	XDG_CONFIG_HOME	HOME/Library/Preferences
Local Configuration	%LOCALAPPDATA%	XDG_CONFIG_HOME	HOME/Library/Application Support

3. A combination of both env vars and a config

In order to accommodate for various config options, zigup might prioritize an env var (e.g., $ZIGUP_HOME) over a config, and if not found, search for a config file, and if not found, use the default.

Discussion

This is obviously my preference, but the actual implementation would require discussion with the community to reach a consensus. Moving the compiler storage dir to $HOME/.zigup/compilers enables storage of other metadata and info in the $HOME/.zigup dir including the index file (allowing for a quick search for available versions), and potentially a settings/config file as specified in 2.1.

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Note: my reasoning for this is that I like to store all zig bins in $HOME/.zig/bin. Cargo stores bins in $HOME/.cargo/bin and rustup toolchains are stored in $HOME/.rustup/toolchains