utility/telescope: custom extensions API

[NotAShelf]

Implements a basic telescope.extensions option to take a list of additional extensions with the following schema:

[
  {
  	name = "name of the extension";
  	packages = [ <package & possible dependencies>];
    setup = {
      <extensionName> = { ... }; 
    };
  }
]

Name is used to construct the load_extension line, packages are added to vim.startPlugins verbatim and all setup attrs are merged into one to be passed to telescope's own setup table.

Fixes #542.

[github-actions]

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Debug Information

Triggered by: NotAShelf

HEAD at: telescope-ext

Reruns: 134