Supported source map features

##Supported

• Source Map v3 "regular" maps (the default format produced by bundlers).
• 1-field (unmapped), 4-field (mapped), and 5-field (mapped with name) segments.
• sourceRoot prefixing.
• null entries in the sources array (returned as sourceFileName === null).
• Inlined sourcesContent.
• ignoreList for third-party sources (exposed via isIgnored()).
• Enclosing scope resolution via scopeAt(). Heuristic polyfill today, natively backed by the ECMA-426 Scopes proposal when bundlers begin emitting it.

##Not supported (by design)

• Writing, generating, merging, or transforming source maps.
• Indexed and sectioned source maps (throws UnsupportedSourceMap. File an issue if you need this).
• Source Map v4 or legacy v2.
• External source fetching. Only inlined sourcesContent is read.