fix(js_formatter): Only parenthesize default-export function cast expressions

[faultyserver]

Summary

Fixes #914. This is the last case of that issue (the middle one).

Default-exported cast expressions for functions require parentheses to disambiguate between a cast expression for the function and a default function declaration export. As an example:

export default function foo() {} as typeof console.log;
// is actually
export default function foo() {};
as;
typeof console.log;
// because the declaration statement ends, and then `as` is considered a new statement

In this case parentheses force the statement to be interpreted as a default export of a cast, rather than a declaration:

export default (function foo() {} as typeof console.log);

But (seemingly) all other kinds of default export expressions are able to be written without parentheses, as Prettier prints them out that way. So this PR implements a new check for when the parent of the cast is an export default and only adds parentheses for function expressions then, omitting them in every other case.

All other types of parent expressions are unaffected.

Test Plan

Added some needs_parentheses tests in as_expressions.rs to ensure these cases are hit correctly.

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[Conaclos]

Looks good to me!