Use forward slash in path for sources

[jmholzinger]

On windows the source map file is generated using windows style pathing. This results in the sources being represented as individual resources in the devtools, rather than nested under a folder structure.

For instance, I would expect this:

{
  ...
  "sources": [
    "..\\src\\index.ts",
  ]
  ...
}

To instead look like this:

{
  ...
  "sources": [
    "../src/index.ts",
  ]
  ...
}

See a similar issue that was reported for the grunt-contrib-concat module:
gruntjs/grunt-contrib-concat#110