Update CONTRIBUTING.md

[lrapoport-cf]

Update CONTRIBUTING.md to capture the following:

• best practices / style guide for wrangler commands
• note that new commands should be implemented API first
• note to include metrics collection logic in new commands
• link to industry JS/TS conventions
• PRs titles should be prefixed with the name of the package the change is for e.g. "[wrangler] add new command"

[lrapoport-cf]

item "best practices / style guide for wrangler commands" completed via #6526

[penalosa]

note to include metrics collection logic in new commands

This was addressed when turning on metrics by default (which automatically added metrics collection to all commands): #7291.

In terms of API-first, over the last couple of years we haven't really enforced that, and Wrangler as an API hasn't been as much of a priority. Given that, I've closed this—we can always re-open if API-ifying Wrangler becomes more of a product priority