Try enforcing contextual type of array literal elements

Part of #19236

Code

const s1: string[] = ["x", 3, null as any];
const s2: string[] = ["x", 3];

Expected behavior:

• s1 should be an error
• s2 should have an error span on 3

Actual behavior:

• s1 is not an error
• s2 has an error span on s2, which makes it impossible to figure out which element is wrong

Hypothesis: When checking a contextually-typed array literal, if an element type isn't assignable to the contextual type, issue an error on that element and return unknownType

Reproduce the two array-literal examples from the issue and inspect the compiler's contextual checking for array literal elements. Done means the first assignment is rejected and the second reports the error on the invalid element 3 rather than on s2.