The tool is added to ./tools/fmt which will be called during
./tools/presubmit as well as by Kokoro during testing.

This requires a new dev container version to bundle the mdformat
tool in the container.

Note: mdformat does not have any IDE integrations, but the prettier
plugin with prose-wrap enabled and a line length of 100 provides
almost identical results.

BUG=None
TEST=./tools/fmt --check

Change-Id: I0ae5659eff8555df7c85c70e62095a1e116b98da
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3416098
Reviewed-by: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Dennis Kempin <denniskempin@google.com>