support streaming in language feature providers (Observable returns)

Created by: sqs

Sourcegraph extensions can now return Observables from provideHover, provideReferences, and other language feature provider methods.

This is useful for:

• Streaming references, especially for external references, which take a lot longer than local references.
• Streaming hovers while the workspace is initializing, so the user doesn't need to keep moving their mouse around on tokens to see if it's ready yet.

Demo videos:

• https://slack-files.com/T02FSM7DL-FE6RZTVJ6-24bbd82b96
• https://slack-files.com/T02FSM7DL-FE6MR49JQ-6169e33377

Example code:

import * as sourcegraph from 'sourcegraph'
import { interval } from 'rxjs'
import { map, bufferCount } from 'rxjs/operators'

export function activate(): void {
    // Normal (non-streaming) provider
    sourcegraph.languages.registerHoverProvider(['*'], {
        provideHover: () => ({
            contents: { value: ' Hello from an extension!' },
        }),
    })

    // "Streaming" hover provider
    sourcegraph.languages.registerHoverProvider(['*'], {
        provideHover: () =>
            interval(100).pipe(
                map(n => ({
                    contents: { kind: sourcegraph.MarkupKind.Markdown, value: `Counter: ${n}` },
                }))
            ),
    })

    // "Streaming" references provider
    sourcegraph.languages.registerReferenceProvider(['*'], {
        provideReferences: doc =>
            interval(500).pipe(
                bufferCount(7),
                map(values => {
                    const lines = doc.text.split('\n')
                    const lineNumbers = values.map(value => (value * 7) % lines.length)
                    return lineNumbers.filter(line => lines[line].length > 3).map(
                        line =>
                            ({
                                uri: new sourcegraph.URI(doc.uri),
                                range: new sourcegraph.Range(
                                    line,
                                    lines[line].length / 3,
                                    line,
                                    (lines[line].length * 2) / 3
                                ),
                            } as sourcegraph.Location)
                    )
                })
            ),
    })
}

TODOs (post-merge):

• Further cleanup in the jsonrpc2 TS directory. We are no longer using standard JSON-RPC 2.0, so we should remove other JSON-RPC 2.0 features we don't use and update docs.