Implement string literal trim() folding

[puddly]

The ECMAScript spec requires implementations at least trim off Unicode 3.0 whitespace characters plus the BOM. I've included all of code points with the White_Space property from Unicode 10.0.0.

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[brad4d]

@puddly

I think this PR looks OK to accept, but we do need the CLA signature to do so.

[puddly]

Alright, I've signed it.

[googlebot]

CLAs look good, thanks!

[brad4d]

imported and sent for internal testing and review

[brad4d]

submitted internally.
should be in Monday's push to GitHub