Guard private syntax preservation by transform profile

[OskarEichler]

Summary:

customTransformOptions.unstable_preserveClassPrivate currently disables private-field and private-method transforms for every profile. With hermes-legacy, the preset still lowers the surrounding class syntax, and Babel then aborts because its class transform requires the private transforms.

Only preserve private syntax when the selected profile also preserves class syntax. Stable and canary Hermes profiles keep their existing experimental behavior; legacy profiles continue lowering private fields and methods together with classes instead of crashing.

Changelog:

[GENERAL] [FIXED] - Keep private class transforms enabled for profiles that lower classes.

Test Plan:

• Added a focused hermes-legacy regression containing both a private field and private method with the preservation option enabled.
• Exact baseline throws Babel's private-method transform error; the fixed preset compiles and emits the normal private-field helpers.
• Full preset Jest passes: 4/4 suites, 111/111 tests, 16 snapshots.
• Fresh Flow check reports 0 errors.
• Targeted no-ignore ESLint, Prettier, and git diff --check pass.

No breaking change: stable/canary preservation is unchanged, while an invalid legacy configuration now compiles correctly.