Diagnostic and Quickfix for \usepackage duplicates and ordering

[nolanking90]

Making this draft PR to gauge interest and get feedback from upstream.

This PR does the following:

• Creates diagnostics to check for duplicate \usepackage statements.
• Creates code actions to remove some or all of the duplicate packages.

This is just a test run of writing code actions. If there is interest, I plan to write a check for conflicting package ordering (e.g. hyperref), and possibly a code action to correct the package order.

Some comments:

• I called these \usepackage statements Imports (i.e. Diagnostic::ImportError) in the diagnostic error enums before I remembered there is an \import command. I will probably rename these enum variants to avoid confusion.
• This probably duplicates some functionality from elsewhere, e.g.to_proto. If you see something obvious, please leave me a comment and I will refactor as needed.