Extension API: add token type provider

Created by: chrismwendt

Prior to this change, hover tooltips (specifically the "Find references" button) would be shown even on comments, whitespace, etc. There was no way for extensions to prevent that from happening.

After this change, there will be a token type provider in the extension API that can be used to prevent hover tooltips from showing up on comments, whitespace, etc. Here's how you will be able to use it:

        const identifier: sourcegraph.TokenType = 'identifier'
        const whitespace: sourcegraph.TokenType = 'whitespace'

        ctx.subscriptions.add(
            sourcegraph.languages.registerTokenTypeProvider(
                [{ pattern: '*.go' }],
                {
                    provideTokenType: (doc, pos) => {
                        // toy example: returns identifier on even lines and whitespace on odd
                        return Promise.resolve(pos.line % 2 === 0 ? identifier : whitespace)
                    },
                }
            )
        )

Test plan:

• Sideload an extension that registers a token type provider that emits identifier on even lines and whitespace on odd lines, make sure hover tooltips show up on even lines and not on odd lines
• Make sure existing extensions still show hover tooltips even though they haven't registered a token type provider

Previous implementation attempt https://github.com/sourcegraph/sourcegraph/pull/3188

TODO

• Add proper docstrings
• Add tests