[Feature] add shorter way to get value from Result

[HosamHasanRamadan]

Adding shorter and more readable way to get value from Result

old

final result = Result.value(10);
if(result.isValue) result.asValue!.value
// or 
result.asValue?.value

new

final result = Result.value(10);
if(result.isValue) result.requiredValue
// or
result.valueOrNull

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[mosuem]

Closing as the dart-lang/async repository is merged into the dart-lang/core monorepo. Please re-open this PR there!