Add ProgressParams

[algesten]

This PR is in flux and shouldn't land before the proposed changes to LS spec.

microsoft/vscode-languageserver-node#261

Fixed feedback in #42

[Xanewok]

Thankfully, microsoft/vscode-languageserver-node#261 has been merged now 🎉
Would it be possible to rebase this and adapt (e.g. title is non-optional) to the changes?

@Marwes how should we proceed with proposed LSP features? Should we typedef this as usual?

[Marwes]

how should we proceed with proposed LSP features? Should we typedef this as usual?

What do you mean with typedef?

[Marwes]

how should we proceed with proposed LSP features?

I am thinking we may want to merge proposed features but put them under a "propesed"/"unstable" feature for which we make no semver promises for (users will need to use = when depending on this crate)

[Xanewok]

Makes sense! @algesten could you update this PR or should I?

What do you mean with typedef?

Sorry, took a mental shortcut - I meant just write type definitions for the LSP types/interfaces as usual

[algesten]

Sorry. I was not monitoring my email there. Seems you solved it! ❤️