fix: make isMatching(p) infer the pattern as a const type parameter

[gvergnaud]

Motivation

Currently, you need to use as const on your pattern to make sure that type narrowing works as you intend:

type Pizza = { type: 'pizza', topping: string };
type Sandwich = { type: 'sandwich', condiments: string[] }
type Food =  Pizza | Sandwich;

declare const x: Food

const isPizza = isMatching({ type: 'pizza' })
const isPizzaConst = isMatching({ type: 'pizza' } as const)

if (isPizza(x)) x  // x: Food
if (isPizzaConst(x)) x  // x: Pizza

This isn't intentional. Th 2-arguments overload of isMatching doesn't have this problem.

This PR fixes that.