This is a backport of numpy#24493 and numpy#24291. The purpose of
this is to ease future backports that expect these files.

- CI: move some jobs in `build_test.yml` to Meson

- CI: split `build_test.yml` into three GHA jobs files

  Also documents better what is being run. See numpygh-24410 for the
  overall restructuring plan for GitHub Actions CI.

- CI: merge `linux_meson.yml` into `linux_blas.yml`

- TST: disable mypy tests in test suite unless an environment variable is set

  These tests are super slow, and they're effectively always passing in CI.
  Running them on all "full" test suite runs is too expensive. Note that
  SciPy has an XSLOW mark, NumPy does not. So use an env var for now.

- CI: add new GHA CI jobs to run MyPy across OS/Python flavors

typing_nptensions -> typing_extensions

Backport of numpy#24625.