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?