Add a null forgiving operator

[jeskew]

Resolves #9565

This PR adds a postfix ! operator that asserts that a given value is not null, which transforms the assigned type of the value from null | <type> to just <type>.

Microsoft Reviewers: Open in CodeFlow

[majastrz]

Exclamation

Can you also update our pseudo grammar file?

[majastrz]

memberExpression ->

Can you also add some parser unit tests to ensure that the new operator associativity is as we expect?

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Refers to: docs/grammar.md:88 in 0d972f7. [](commit_id = 0d972f7, deletion_comment = False)

[majastrz]

TestMethod

Should we also update some of the baseline tests to add the new operator?

[jeskew]

I added a usage to the TypeDeclarations_LF baseline (mostly because an array of a union type is the easiest way to trigger a nullable type prior to #9533 or #9454 landing), and it looks like one of the invalid expressions from InvalidExpressions_LF is now valid (at least at a syntactic level).

[majastrz]