* fix: Allow registering fields with reserved property names like 'constructor'

Fixes #489

Problem:
JavaScript object property access using bracket notation (fields['constructor'])
conflicts with built-in Object.prototype properties like 'constructor',
'toString', '__proto__', etc. This made it impossible to register fields
with these names.

Root Cause:
The state.fields object was initialized as a plain object literal ({}),
which inherits from Object.prototype. Accessing fields['constructor']
would return the Object constructor function instead of allowing a custom
field to be created.

Solution:
1. Initialize state.fields using Object.create(null) to create an object
   without a prototype chain
2. Use Object.assign(Object.create(null), ...) when spreading fields to
   preserve the null prototype
3. Add comprehensive test coverage for reserved property names

Changes:
- src/FinalForm.ts:
  - Line 192: Change fields: {} to fields: Object.create(null)
  - Line 254: Use Object.assign(Object.create(null), ...) in renameField
- src/FinalForm.registerField.test.ts:
  - Add test for registering 'constructor' field
  - Verify change, blur, focus handlers work correctly
  - Verify unregister works properly

Impact:
✅ Allows fields named 'constructor', 'toString', etc.
✅ No breaking changes - Object.create(null) is fully compatible
✅ No performance impact
✅ Backward compatible with existing code

* Fix #437: Preserve submitErrors when sync validation fails

When submitting a form that has both sync errors (field validation) and
previous submitErrors (from last submit), the submitErrors were being
cleared unconditionally and then never regenerated because submission
was prevented by sync errors.

Root cause: submitErrors were deleted BEFORE checking hasSyncErrors(),
so if sync validation failed, the function returned early without ever
populating new submitErrors.

Solution: Only delete submitErrors AFTER confirming there are no sync
errors, i.e., only when we're actually proceeding with submission.

Changes:
- Moved 'delete formState.submitErrors/submitError' to after the
  hasSyncErrors() check
- Updated test to expect the new correct behavior (submitErrors
  preserved when validation fails)

All existing tests pass (339/340, 1 pre-existing failure unrelated).

* fix: Remove package-lock.json and revert unrelated Object.create(null) changes

Address Richard's review feedback:
- Remove package-lock.json (we use yarn)
- Revert Object.create(null) changes (unrelated to #437)

Keep only the core fix: preserve submitErrors when sync validation fails

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Co-authored-by: Erik Rasmussen <[email protected]>