Make external interfaces compatible with xmldom

[tmcw]

• Fixes Why is @types/geojson a required dependency? #132
• Fixes Incompatible type with xmldoc #133

This resolves two fairly long-standing issues. It does two things:

1. Removes @types/geojson and @xmldom/xmldom as peerDependencies. If you want to use them and their types with TypeScript, you install those packages separately.
2. Adds a union type for all Document parameters that go into the module to signal compatibility with xmldom.

[tmcw]

This doesn't work, if xmldom is not available, then this lets the methods accept any.

[tmcw]

If anyone knows the answer here, let me know: is it possible to expose an index.d.ts file that tries to use types from an optional dependency, but if that module is not present, treats its types as never?

[tmcw]

Okay, I think this is roughly the path as-is. There's no optional-type-if-you-have-it-installed construct afaict, so if you use this module + typescript, you'll need xmldom/xmldom and types/geojson.

Testcase for both kinds of issues:

import { kml } from "@tmcw/togeojson";
import { DOMParser as XDOMParser } from "@xmldom/xmldom";

const x = new DOMParser();

const d = await fetch("hi")
	.then((r) => r.text())
	.then((r) => x.parseFromString(r, "text/xml"));

const x2 = new XDOMParser();

const d2 = await fetch("hi")
	.then((r) => r.text())
	.then((r) => x2.parseFromString(r, "text/xml"));

const y = kml(d);
// this should have an accurate features type
y.features;
const y2 = kml(d2);
// This should error
kml(1);