Support 'go to type definition' code action

[Giesch]

The LSP spec defines a 'Goto Type Definition Request'; it allows going from a variable to the type definition (rather than variable declaration). While it's not currently supported by the Elm LSP, it seems similar to other features that are supported. I'd be interested in taking the time to add it, if that's welcome. Would /common/providers/definitionProvider.ts be a good related example to start from?

This is related to the conversation on #633, but I'm personally not a VS Code user - this issue is for the LSP feature.

[razzeee]

Yeah, definitionProvider should be a good template