Have tools/devtool fmt also run black and mdformat

Previously, it would only run `cargo fmt`. Now, it will please all
formatting rules we have in our CI, e.g. also include python and
markdown formatters.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

tools/devtool

@@ -861,6 +861,8 @@ cmd_test_debug() {
 #
 cmd_fmt() {
     cmd_sh "cargo fmt --all -- --config $(tr '\n' ',' <tests/fmt.toml)"
+    cmd_sh "black tests tools .buildkite"
+    cmd_sh "mdformat $(git ls-files '*.md' | tr '\n' ' ')"
 }
 
 cmd_mkdocs() {