Allow redundant trailing `;`s in the mappings to be omitted

[sapphi-red]

If I understand correctly, trailing ;s in the mappings does not have any information other than the generated file contains those lines. In that case, I think those trailing ;s can be omitted.
The current text does not describe whether the trailing ;s are required or can be omitted.
https://tc39.es/ecma426/#sec-mappings

[nicolo-ribaudo]

While the grammar does not explicitly say that trailing ;s can be omitted, the decoding algorithm (https://tc39.es/ecma426/#sec-DecodeMappingsField) generates the same decoded data structure for something;else and something;else;;;;, thus making them equivalent.

(the algorithm currently has a bug that makes it not well-defined for lines with no mappings, I opened #230 to fix it)

[sapphi-red]

Ah, I see.

[jridgewell]

I agree that trailing lines don't make sense from a consumer's perspective. But I don't like that this means we can't properly round-trip. The array-per-line approach I take with gen-mapping and trace-mapping is carries both mappings data and line count data.