Move tessellation shader files into a subdirectory

Also renames GrStrokeShader -> GrStrokeTessellationShader.

Bug: skia:10419
Change-Id: I75b09f794ef86a4cd8c467a1fd61cc87fe68091b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413756
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
27 files changed