Code submission and review fx subcommands

Command Description
doc-checker Check the markdown documentation using a variety of checks.
mdlint Markdown linter
cl Print the gerrit review URL and attempts to open it in the user's default browser.
g-review Perform a Gemini review on the current commit
gerrit-split-cl Submits chains of CLs to Gerrit
gerrit-submit Submits chains of CLs to Gerrit