fix(copilotchat-nvim): use custom prefix from astro core if defined

[fynnfluegge]

📑 Description

With this PR it is possible to define the prefix for CopilotChat.nvim via AstroNvim/astrocore opts.options.g.copilot_chat_prefix or if not defined use <leader>P as default.

[github-actions[bot]]

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

• [x] 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

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

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

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

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