Implements a basic decoder for utf8 and codepoint tables for XID_Start and XID_Continue.
The first steps to supporting unicode identifiers.
Bug: tint:1437
Change-Id: I7371d1e63d664f3a3c34929221c7c6d043beb45d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80842
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>