Define mappings that can be ignored when generating source map

[jridgewell]

Occasionally, a mapping is generated that contains essentially the exact same information as the mapping immediately before it:

CCCC,AAAA

This defines two mappings, both of which point to the same exact spot in the original source file. This isn't necessary, and adding the second just bloats the source map.

I've defined a couple of more "useless" mappings in https://github.com/jridgewell/gen-mapping/blob/7626d310002df682012dc29bdfa4d674027f1538/src/gen-mapping.ts#L382-L418, which is exposed to the generating tool through maybeAddMapping (Babel and Terser use this). It'd be good to formally define which mappings are duplicates and can be removed when generating a source map.

[sjrd]

The source map writer of Scala.js contains similar logic:
https://github.com/scala-js/scala-js/blob/main/linker/shared/src/main/scala/org/scalajs/linker/backend/javascript/SourceMapWriter.scala#L148-L174