Allow narrowing for any left-hand in operand - #45152

Extends a more restrictive fix already in place for #42639.

Previously, narrowing of an in expression happens if the expression satisfy two constraints:

• In the binder: isNarrowableInOperands is a syntactic constraint that allows narrowing of an expression if the right in operand is a narrowing expression, and the left in operand is a string literal (e.g. if ('prop' in c) { ... }).
• In the checker: narrowBinaryExpression only calls narrowByInKeyword if the left in operand's type is a string literal type.

An already merged PR (#44893) relaxed the binder constraint to allow narrowing in cases the left in operand is an identifier, to address scenarios such as:

type A = { a: number };
type B = { b: string };
declare c: A | B;

if ("a" in c) {
    c; // c has type `A` here
}

const a = "a";
if (a in c) {
    c; // c should also have type `A` here; previously it had type `A | B`
}

This PR further relaxes the constraint in the binder for narrowing in expressions, by no longer placing any syntactic constraint on the left in operand expression. This means that an in expression is considered narrowable by the binder if the right expression is narrowing:

type A = { a: number };
type B = { b: string };
declare c: A | B;

if ("a" in c) {
    c; // c has type `A` here
}

const a = "a";
if (a in c) {
    c; // c has type `A` here
}

let stringA: string = "a";
if ((stringA as "a") in c) {
    c; // c now has type `A` here too
}

if ((stringA as ("a" | "b")) in c) {
    c; // c has type `A | B`; not narrowed because the left expression has type `"a" | "b"`, i.e. not a string literal type
}

Concerns:
From what I can tell, a possible concern would be performance, since this PR makes it so the checker attempts to narrow more types of in expressions, but the impact may be negligible.
A caveat here (brought up by Andrew Branch (@andrewbranch)) regarding perf tests is that, since the checker currently doesn't narrow those kinds of expressions, real world code is unlikely to have those kinds of expressions.