Clarfiy what it means to be an alphanumeric identifier

[lilleyse]

Follow up to CesiumGS/glTF#60. Includes more strict definition for "alphanumeric identifier".

[javagl]

I wondered whether it could be worth moving the block from ID into an own paragraph like "Identifiers", explain the regex and further constraints there once, and then just link to that from every ID section.

(I could create a draft PR on top of this one, just to see how this could look like)

Also, I wondered if there should be any constraints for Semantic names (maybe not even at https://github.com/CesiumGS/3d-tiles/blob/637e53f2d0af1968b1e0a60a0863fbe2debb2759/specification/Metadata/README.md#semantic , but directly in the semantics docs). The constraints that are described there verbally could be captured in a RegEx as well. (An aside: I thought there was a requirement for "custom" semantics to start with _, but I didn't find it right now...?)

[lilleyse]

I wondered whether it could be worth moving the block from ID into an own paragraph like "Identifiers", explain the regex and further constraints there once, and then just link to that from every ID section.

(I could create a draft PR on top of this one, just to see how this could look like)

Sure, go for it!

[lilleyse]

@javagl and I reviewed each other's changes here, merging.