Suppress script output based on ENV

What does this MR do and why?

In an attempt to operationalize this script and add it to the small collection of lefthook-invoked scripts, we need to emit "proper" exit codes. At the same time, we want to be able to still run the script manually, in order to generate a list of known unused methods. This MR:

• checks the number of unused methods detected, and emits an error or warning message when the number differs from the last known count
• uses an ENV to suppress output to STDOUT (defaulting to hiding this output)
• adds proper exit codes

Related to #444243

Screenshots

Error	Warning	List