Add support for SourceMap emit and baselines

[rbuckton]

This wires up source map emit to the emitter and enables source map baselines. After some fixes, the few source map diffs seem to be mostly due to missing transforms.

This also makes a minor change to the ast.SourceFile type to make Text() a method rather than a field. This was partially done to support sourcemap.Source, but also to make it easier to access the source file text while debugging as the Go debugger gets confused by Text being a field on SourceFile but a method on Node, which is observed when adding file.Text as a Watch expression or when referencing it in the Debug console.