Move modifiers-declaration support out of DSLCore.

The error-handling and IRNode-creation portion of the logic has
moved into the ModifiersDeclaration class. The remainder now lives
in the Parser.

Change-Id: I53ed44f732ad56d74af4247243a1852d74af1715
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/695301
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
11 files changed