[infra] Add building Mesa to CMake builds

Adds a generic Meson target, and then uses that for the Mesa build.

Including this target is guarded by the new TINT_BUILD_MESA flag and
the tint_wgsl_fuzzer target, so that none of this is included/needed
if an embedder/dev isn't using it.

Note: There is a number of hardcoded values, like the options being
passed to the Mesa build and the relative paths between the Mesa and
Meson checkouts. Making these configurable will be part of the work of
separating the mesa/meson build rules out into their own repos for
other projects.

Bug: 475840954
Change-Id: I4f39fe5cc7f5a2a7b40c769ecc196cdabd1ec9b4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/289795
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>