[WIP] "Replace with type synonym" action

[mpardalos]

Adds an action to replace instances of the RHS of type synonyms in a type with their equivalent type synonym.

Also adds some tests for the retrie plugin.

Addresses #506

[pepeiborra]

Can you attach some description and/or screenshots showing how this feature works?

The retrie plugin already offers code actions to fold and unfold type synonyms, isn't that what you want?

[mpardalos]

I don't yet have the feature working, I just opened this PR to declare I'm working on the issue, once I have some screenshots I will attach them here. I can close the PR and reopen once the feature is actually working if that is how it's usually done.

This PR is closely related to that feature but it's about adding that hint to the use site, in order to fold one instance of a type synonym. It should be useful for synonyms that come from libraries.

[mpardalos]

Part of the work of the feature is done. You can apply type synonyms at single usage sites.

What remains is to only suggest relevant type synonyms. We are currently suggesting any type synonym in scope, even though the relevant action does nothing.

[mpardalos]

The plugin will now only suggest relevant type synonyms:

What remains now is to include suggestions for imported type synonyms, which would probably be the most useful.

[michaelpj]

Closing as dead.