feat: extract type defs to assert.d.ts

[mxdvl]

What does this change?

Moves the type definitions of assertions into the main declaration file, under the LHCI.AssertResults namespace.

Why?

This allows the output JSON file to have an expected type. This is especially useful in Deno, where you can do:

// With a version number that’s not actually v0.9.0
import "https://raw.githubusercontent.com/GoogleChrome/lighthouse-ci/v0.9.0/types/assert.d.ts";

const results: LHCI.AssertResults.AssertionResult[] = JSON.parse(
	Deno.readTextFileSync(`.lighthouseci/assertion-results.json`)
);

[connorjclark]

Seems cool to me. Thanks.