add path mapping

[llgcode]

Hi @roblourens
this pr fixes #146 and have to be merged with microsoft/vscode-chrome-debug#324
I added pathMapping in the launcher config:
So it replace webRoot and sourceMapPathOverrides that are no longer needed to configure a launcher
I've tested with some typescript and sourcemap and it seems to work fine with just a pathMapping.

It was tricky to configure the debug environment but I finally installed projects and found the right procedure for launching.

[msftclas]

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[msftgits]

Hi, I am closing and re-opening this PR to bump the CLA bot. Sorry for the inconvenience!

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[roblourens]

This is great, thanks for the PR! I don't think I want to deprecate webRoot, since it's simple and almost always the only thing someone needs. But I'll merge and test and release this when I have a minute.