[API] Expose some type checker features that were available in TS6

[dragomirtitian]

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

Add 4 checker features that were available in TS6 and that we are using for out docs system:

• UnionType.getOrigin() — returns the pre-normalization union (e.g. recovers named unions collapsed during flattening) - This was marked as internal in 6.0, but it is useful for the doc system to know where the type is coming from.
• Checker.symbolToString(symbol, enclosingDeclaration?, meaning?, flags?) — exposes checker's symbol formatting; adds SymbolFormatFlags enum.
• UniqueESSymbolType.escapedName + Type.isUniqueESSymbolType() — exposes escaped name for unique symbol types.
• ConditionalType.getInferTypeParameters() — exposes type parameters bound by a conditional type's infer clause.