[General] Fix DiagonalDirections type

[j-piasecki]

Description

It seems like TypeScript isn't treating an expression where every node is const as const:

const a = 1; // a: 1
const b = 2; // b: 2
const c = a + b; // c: number

This caused DiagonalDirections type to be a number instead of a union of consts.

Test plan

Check the inferred type of DiagonalDirections