v5: remove deprecated checkOptions and checkinitialData exports

[tyler-reitz]

Background

checkOptions and checkinitialData (in src/index.ts) are no longer used internally as of #740, which tightened ReactFireOptions generics and rewrote checkIdField to read options?.idField directly.

Removing them is a breaking change for any consumer importing them (an ESM import error at load, not just a type change), so per review on #740 we're keeping them as dead exports for now and deferring removal to v5.

To do in v5

• Remove checkOptions from src/index.ts
• Remove checkinitialData from src/index.ts
• Regenerate reference docs (npm run docs:fork) to drop docs/reference/functions/checkOptions.md and checkinitialData.md

Notes

• checkIdField stays (still used by the Firestore data hooks).
• Side effect worth noting: after the fix: tighten ReactFireOptions generic types, remove T | any widening #740 tightening, checkinitialData's inferred return type is now unknown (was any). Harmless while it's unused, but another reason to retire it.

Context: #740.