[scopes] [SourceMap] show mapped names in scopes

[jasonLaster]

It would be great to map minified variable names to original names

{
generatedColumn:45
generatedLine:1
name:"current"
originalColumn:27
originalLine:3
source : "sequence_print.js"
}

Chrome

Firefox

[jasonLaster]

The source map env spec outlines what we'll need to do in the general case

https://github.com/fitzgen/source-map-rfc/blob/scopes-and-bindings/proposals/env.md