Resolve relative source maps and honor sourceRoot

[chrisdp]

In SourceMapManager.ts, a relative sourceRoot was passed directly to path.resolve(), which resolves it against process.cwd() instead of the source map file's directory.

Fix (two changes):

• Resolve sourceRoot against path.dirname(sourceMapPath) first, so relative sourceRoots work correctly — e.g. ../../src in a map at pkg/components/foo.brs.map now correctly resolves to src/ at the project root.
• Clear parsedSourceMap.sourceRoot after resolving, so SourceMapConsumer doesn't try to re-apply it on top of the already-absolute sources paths.In SourceMapManager.ts:72, a relative sourceRoot was passed directly to path.resolve(), which resolves it against process.cwd() instead of the source map file's directory.

[TwitchBronBron]

Looks good to me, once you've tested and verified this works, it's good to merge