constructor(file: string) {
    this._file = file;
    this._sections = [];
    this._line = 0;
    this._column = 0;
    this._code = '';
    this._afterMappedContent = false;
  }
Builds a source-mapped bundle by concatenating strings and their
corresponding source maps (if any).

Usage:

const builder = new BundleBuilder('bundle.js');
builder
  .append('foo\n', fooMap)
  .append('bar\n')
  // ...
const code = builder.getCode();
const map = builder.getMap();