fix: attribute location

[jorgsowa]

Fixes #1125

1. attribute start position was wrong — this.node("attribute") was called after consuming the name and args, so start = end (both captured at the post-read position). Fixed by capturing the node before reading anything.
2. attrgroup end position was wrong — this.node("attrgroup")([]) was created and finalized immediately (before reading attrs or consuming ]), so start = end. Fixed by deferring finalization until after ] is consumed, giving the node its correct span.

[alexander-akait]

@jorgsowa let's rebase