Integers at or above 1e21 (and Number.MAX_VALUE) stringify via
Number.prototype.toString(10) in exponential notation, e.g. 1e21 -> "1e+21".
The int tags identified these numbers (1e21 % 1 === 0) and represented them
with that exponential text, producing output like `!!int '1e+21'`. That text
matches no `!!int` resolution pattern, so loading the library's own dump output
threw "cannot resolve a node with !<tag:yaml.org,2002:int> explicit tag".

Per YAML 1.2.2 the Core (10.3) and JSON (10.2) `!!int` resolution accepts only
[-+]?[0-9]+ (plus 0o/0x forms); exponential text is a float, not an int.

Make the int tags reject numbers whose decimal stringification contains an
exponent, and let the float tags claim them instead. The float `represent`
already emits a valid float form (1.e+21) for such values, so they now
round-trip. Restores the "numbers survive round-trip" invariant (#737) for
large integers across the default, Core, JSON and YAML 1.1 schemas.

…rify round-trip test (ref #771)