feat: Add transformer for links

[BURG3R5]

Description

I've implemented a transformer that lets users insert titled links with the usual [title](url) format. I've also added partial tests for the transformer.

Additional context

Currently the PR is a Draft since I'd like to implement a Mod + K shortcut too, but that seems difficult right now (accessing the clipboard is async while AFAIK commands need to be synchronous). I'll look around for a bit, but if nothing else, at least the transformer is working.

Edit: No longer a draft!

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[BURG3R5]

There's this discussion, but the links are dead. And web.archive.org is currently down so I can't check if they've been archived.

At present, I'm exploring this example code.

[BURG3R5]

I've tried out the code from that repo, and while it technically works, it's just too unwieldy UX-wise as well as a major decision product-wise (adding a 'menu'), so I'll leave it for now.

[BURG3R5]

@FPurchess Could you review this PR?