Directives for src/tint

Instructions that are useful for development for files under src/tint.

Testing

• When told to run tint end2end tests, or tint e2e tests, unless otherwise instructed, run these tests with tools/run tests --tint {PATH_TO_TINT}.

Modifying build files

• When source files are added, deleted, renamed, or moved, do not update BUILD.gn, BUILD.cmake, or BUILD.bazel files directly. Instead, run ./tools/run gen to update these files.