This will be used by the upcoming DSLParser, which needs to be able to
put DSLExpression and DSLVar into containers such as std::vector and
std::optional.
Change-Id: I8d367cfd0b3a852a368c69a5b3be6c0eaa41d74a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404156
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
