Services Request: Treat `_`-prefixed bindings as internal: skip them in declaration rendering and the binding check

[Kheartz]

Problem

Mods may want to mark dev/internal bindings with a leading underscore (e.g. Pvp._staggerRun) for internal/private automated testing (etc...). services-cli has no way to keep them out of the public contract or the rendered docs:

• docs check scans bindingSources for bound names and fails with undocumented_bindings if any name is missing from the declarations. So a mod can't remove these bindings before the check.
• renderDeclarations writes every symbol, function, and property from the metadata into generatedEntryPoint, and its JSDoc output has no way to add @internal or @hidden.
• As a result, internal bindings end up in the published api.d.ts, the TypeDoc reference, and public/api/<target>.json.

Seen in 0.6.3 and 0.8.0.

Proposal

Treat a binding whose JS name starts with _ as internal:

1. Binding check: still collect _ names, but don't require them in the declarations.
2. Declaration rendering: leave _ functions, properties, symbols, and variables out of the generated .d.ts.
3. Opt-out: add an optional per-target setting (e.g. "internalPrefix": "_", or null to turn it off) for mods that want them kept.

Acceptance criteria

• A metadata function named _foo doesn't appear in the generated declarations or the rendered reference.
• docs check passes when _foo is bound in C++ but not declared.
• A misspelled public binding (no leading _) still fails docs check.
• Mods without _-prefixed bindings get the same output as before.