Make some items extensible in the grammar

[hbenl]

To allow future versions of the standard to extend OriginalScopeStart, OriginalScopeEnd, GeneratedRangeStart and GeneratedRangeEnd (and possibly GeneratedSubRangeBinding and GeneratedRangeCallSite), an optional VlqList should be added to the grammar at the end of these items, otherwise a parser implementing the current version of the grammar would reject future sourcemaps that make use of such extensions.

[rakudrama]

I don't think this is necessary, but a limited application of this idea would be ok.

OriginalScopeStart and GeneratedRangeStart items have a flags Vlq which is necessary to disambiguate which optional elements are present. So for these items, it seems at first reasonable to permit 'unassigned' Vlqs.

Other items use the number of Vlqs to determine meaning, so I would prefer that these remain non-extensible.
A clear example is GeneratedRangeEnd where the line is optional, detected by number of Vlqs.

There also needs to be some specification about how those Vlqs are to be interpreted in the future.
The main constraint is that any new Vlqs cannot affect the existing accumulators.