Add prompt library builder for codecompanion.nvim extension

[Davidyz]

This PR provides a streamlined experience of indexing and querying the neovim runtime (including the lua source code/type stub and the help doc).

Specifically, this PR registers a new prompt that:

• Automatically vectorises the neovim runtime files ($VIMRUNTIME), including *.lua and *.txt.
  • Support pre_hook option in prompt library when applicable for chat strategy. olimorris/codecompanion.nvim#1964
• Provides a pre-defined system prompt that tells the LLM to query from the nvim runtime collection to answer the user's question

I'm also interested in turning this into a prompt builder so that you can easily do the same for an arbitrary local directory. Done. Users can now easily set up a custom prompt in the action palette that is similar to the built-in one.

Demonstration

The following will create a collection for the kitty documentation that comes with the Arch Linux package for the kitty terminal emulator:

require("codecompanion").setup({
  extensions = {
    vectorcode = {
      prompt_library = {
        ["Kitty Assistant"] = {
          project_root = "/usr/share/doc/kitty/",
          file_patterns = { "**/*.txt" },
        }
      }
    }
  }
})

And you'll see a new action in the :CodeCompanionActions menu:

Some default user prompts have been populated in the chat buffer:

You'd just need to type your question here, and the LLM will now use the tool to query from the correct database to answer your questions:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (4580c45) to head (db9f17e).

Additional details and impacted files

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[Davidyz]

Alternative for pre_hook (in case the PR's not accepted): hook into codecompanion's events for chat initialisation.

nvm. Just waiting for the pre_hook to be released now.