Don't enable class features by default in estree

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #12864
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The problem was that:

• the estree parser option means "give me an estree-compatible AST"
• the estree spec got the definition for class fields and private elements
• we updated the estree parser option to produce them
• this breaks every tool which is not aware of the new estree nodes
• technically that's ok, because that's exactly what the estree option is for, but this breaks every older version of @babel/eslint-parser or babel-eslint

I put this behind a new option, to revert the regression. Technically this is a breaking change compared to 7.13.0, but semver itself recommends reverting accidental breakages to restore the previous working state.

[babel-bot]

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