Implement some of Scala Metals' code actions

Pretty open ticket. [Link to Metals' CodeActions section](https://scalameta.org/metals/docs/codeactions/codeactions)

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Here is a code action that seems like low-hanging fruit!


We could have a similar action, where if an upper-case symbol is undefined, we generate a template for a datatype with that name.

Start with the linked Metals CodeActions documentation and compare its CreateNewSymbol example with the Haskell language server's existing code-action support. Define the scope for handling an undefined upper-case symbol and generating a datatype template, then identify the relevant implementation and tests before making changes. Done means the action is available for the stated case and its behavior is covered by tests.

Written by the indexing model from the issue text.