fix: breakpoints in extension subdirs on windows not working (#184195)

It did not normalize to forward slashes, so the empty sourcemap was
created with a source called `foo\\bar.js`. Then the build process was
adding a new source `foo/bar.js` since it didn't see it already in the
sourcemap, but `foo\\bar.js` was later normalized. This normalized
version came first in the mapping, but didn't have any mapped locations,
so breakpoints never got set.

build/lib/util.ts

@@ -219,7 +219,7 @@ export function loadSourcemaps(): NodeJS.ReadWriteStream {
 					version: '3',
 					names: [],
 					mappings: '',
-					sources: [f.relative],
+					sources: [f.relative.replace(/\\/g, '/')],
 					sourcesContent: [contents]
 				};