Scopes: Use unsigned VLQ (1-based) for binding expressions

[szuend]

As discussed in the meeting of 2025-04-24, this PR changes the binding expressions from a signed VLQ to a unsigned VLQ. To signify a variable is unavailable in a given generated (sub) range, we use 0 instead of -1 and the indices themselves are now 1-based instead of 0-based. This is necessary to free up 0 as the "unavailability marker".

To capture some of the points of the meeting here (also available in the notes):

• Using relative indicies into the names array would require us to sacrifice an additional bit to signify "unavailability".
• Another option would be to use 0 as the marker and make positive relative indices "off by one". E.g. "0" means a variable is unavailable, "1" means a variable uses the same binding expression as the previous variable and "2" means it uses the next entry in "names".
• Both of the above options show some minor improvement in the uncompressed size of the scopes encoding but are actually worse after gzip/brotli compression.
• This leads us to this PR as to not overcomplicate the encoding for no gain.

[szuend]

@nicolo-ribaudo adding you explicitly since you didn't have a chance yet to express your opinion. I remember you floated the idea of using B (the minimum possible integer) to mark "unavailability" of a variables' value. We didn't discuss that option though in yesterdays' meeting.