Support better stack trace with sourcemaps

[habibrosyad]

This PR is a continuity of #773 by adding test and fix found issues.

Closes #117
Closes #773

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Tests

• Run the tests tests with npm test or yarn test)

[benmccann]

thanks so much for picking this PR up!

[benmccann]

I'm not really sure what the point of the file_cache is if it's cleared everytime we try to source map an exception

[habibrosyad]

I noticed that in a stack trace, sometimes you can hit the same file with different lines and cols. The file_cache there is just a means to avoid rereading that file multiple times in a single call to sourcemap_stacktrace.

[benmccann]

this looks good to me

[benmccann]

@habibrosyad this PR will need to be rebased

[habibrosyad]

@benmccann done...

[antony]

This is brilliant. Makes diagnosing server-errors easy. Thank you!