feat(pack): add elixir lang pack with expert

[vonhyou]

📑 Description

Adds a new elixir language pack with the official elixir-lang/expert language server.

• Adds elixir, heex, and eex Treesitter parsers
• Adds Expert LSP via Mason (mason-tool-installer)
• Adds mix format formatter via conform.nvim

📖 Additional Information

Expert is currently at v0.1.0-rc. It is functional for day-to-day use but noted in the README as release candidate stage.

Since Astro v5 still pins nvim-lspconfig to ~2.1, Expert is registered manually via lspconfig.configs. This workaround can be removed once AstroNvim upgrades its lspconfig pin

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).