Remove starts_with and ends_with from SkSL::String.

std::string::starts_with will be available in C++20. In the interim,
prefer using a free function in skstd. This puts us a small step closer
towards removing SkSL::String.

Change-Id: I8c6b33d94c51a643d8cb99ac4c4b1c0556cb9170
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502782
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed