Require plugins to declare their diagnostic codes

Right now diagnostic codes are completely unrestricted. You can use numbers, strings, or a mix. This presents several problems:

• no detection for diagnostic collision. 2 plugins could use the same diagnostic code for completely independent things
• no detection for a typo in your bs:disable comments

This is a breaking change, as we will start flagging all unknown diagnostic codes that are provided through validation flows, as well as in bs:disable comments.

Proposal

1. Add a plugin lifecycle hook (e.g. onRegisterDiagnostics / activation event) where plugins declare the diagnostic codes they own, up front.
2. Support re-declaration if a plugin's diagnostics change based on config (call the hook again on config change, or let plugins return a fresh list each time it's invoked. (e.g. program.registerDaignosticsForPlugin(pluginName: string, diagnosticDeclarations: []) )
3. BrighterScript validates all registered codes at startup/reload and throws on collision.
4. Once codes are registered, bs:disable comments referencing an unregistered code become a diagnostic themselves (typo detection).

Open questions

• Prefix delimiter/format (:, -, .)?
• Namespacing policy: should we require all plugin diagnostics to be prefixed (myplugin:1000), full stop? Core BrighterScript codes stay unprefixed. Or, we could do "prefix only if you collide"?

Start by tracing plugin registration, validation flows, and bs:disable comment handling described in the issue. Resolve the open questions about declaration hooks, re-declaration, namespacing, and delimiters before implementation. Done means registered diagnostic codes are validated for collisions and unknown codes are reported consistently.