Numeric enums as key types in mapped types

[ahejlsberg]

This PR fixes a minor issue related to numeric enums as key types in mapped types. We now generate a numeric index signature when a numeric enum type occurs in the key type of a mapped type. Previously we wouldn't generate anything which was inconsistent.

The PR also fixes a crash that is triggered by the following code:

declare enum E { ONE, TWO, THREE = 'x' }
const e: E = E.ONE;
const x: E.ONE = e;

Fixes #31771.