Support several default sourcemap loaders

[jridgewell]

Eg, a transformationChain, where a series of transformations on a single file are remapped to a root sorucemap

[jridgewell]

Or a builder API:

// Given 3 transforms on a file x.js:
const builder = new resorcery.Builder(finalSourcemap);

const secondMap = builder.traceSource(0, secondSourcemap);

const firstMap = secondMap.traceSource(0, originalFile);

etc.