[Feature] `Rocks install` API for `install_args`

[mrcjkb]

For instance, allowing :Rocks install something opt=true ft=markdown and feeding that directly into the toml.

We already do that[^1] :wink:

[^1]: As long as the extensions know how to parse args that rocks.nvim doesn't know about. So far, only rocks.nvim and rocks-git.nvim know how to parse args.

What could be tricky is coming up with a good API for args that are lists:

:Rocks install {rock} opt=true install_args=["GIT2_DIR=/some/path","foo","bar"]

or

:Rocks install {rock} opt=true install_args=[GIT2_DIR=/some/path,foo,bar]

or, assuming any args that rocks.nvim can't parse are install_args:

:Rocks install {rock} opt=true GIT2_DIR=/some/path

The last one is more concise, but also more brittle: If someone passes in ft=markdown, it would be added to the install_args as long as rocks-lazy.nvim doesn't register an arg parser.

should we instead support an [env] section in the toml, which would modify vim.env?

Luarocks won't pick up GIT2_DIR from an environment variable. It has to be passed as an arg to install.

Since this is more of an edge case, I think we don't need to prioritise passing install_args via :Rocks install initially.

Originally posted by @mrcjkb in https://github.com/nvim-neorocks/rocks.nvim/issues/439#issuecomment-2200585803

[mrcjkb]

Since this ought to be an edge case, we may not want to support this, as it adds quite a bit of complexity.