Fix SourceMapper typing

[bryanhuhta]

When using the SourceMapper in a TypeScript project, the following error occurs:

Type 'import("/Users/bryan/grafana/pyroscope-nodejs/dist/cjs/sourcemapper").SourceMapper' is not assignable to type 'import("/Users/bryan/grafana/pyroscope-nodejs/node_modules/@datadog/pprof/out/src/sourcemapper/sourcemapper").SourceMapper'.
  Types have separate declarations of a private property 'getMappingInfo'.ts(2322)

The critical piece being:

Types have separate declarations of a private property 'getMappingInfo'.ts(2322)

This comes from this method:

pyroscope-nodejs/src/sourcemapper.ts

Lines 232 to 237 in 4c286e7

	private getMappingInfo(inputPath: string): MapInfoCompiled | null {
	if (this.infoMap.has(path.normalize(inputPath))) {
	return this.infoMap.get(inputPath) as MapInfoCompiled;
	}
	return null;
	}

The cause is that we can't use the @pyroscope/nodejs SourceMapper type in place of the @ddog/pprof SourceMapper type because private methods aren't considered when matching type shapes in TypeScript. See microsoft/TypeScript#7755 (comment) for an example and explanation.

As a result, we should not expose the @ddog/pprof SourceMapper type in the PyroscopeConfig type. Rather, we should expose our own implementation of SourceMapper then do a conversion to the DataDog type internally.

The critical change is this method:

https://github.com/grafana/pyroscope-nodejs/pull/101/files#diff-068acd199ee4e6261a4645ae640daf8a4f48c526e60caf6bf55a3ae770faadd9R126-R134

[simonswine]

LGTM