The precise keyword is used in tessellation shaders but introduced in
this extension.

EXT_gpu_shader5 introduces a handful of features to shaders.  This
change only implements the `precise` keyword.

Bug: angleproject:3569
Change-Id: I2252b79098eb8ba2d2faa040d7eaed7548b7051e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1939851
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>