Convert appendf and vappendf to static methods.

This will make it easier to migrate from SkSL::String to std::string.
These methods can then continue to exist as free functions.

Change-Id: I9f6799788aaf42f4a95c6df03d01f9e123ae52c6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502783
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
10 files changed