Intl: Add coverage for ToIntlMathematicalValue constraints

[gibson042]

Fixes #4933
Extends and replaces #4780

• Tests against current spec text rather than Normative: Increase limits on Intl MV and explicitly limit significant digits ecma402#1022
• Adds more context, in particular showing use of RoundMVResult only to identify special values (and not e.g. to round over-precise in-bounds values)
• Removes the presumption of BigInt support
• Covers both sides of the treat-as-infinite transition
• Includes literal decimal/hexadecimal/binary/octal string values (along with their derivation)
  • If we do extend to 1000 decimal digits, I expect such literals to be replaced with string operations like repeat and/or replace