Important changes of the v0.18 version:
- The previous libxmljs-tests.ts file used a require, thus made any
variable from the imported 'libxmljs' typed 'any'. Hence, the test file
actually tested no type declarations at all.
This has been fixed via an ordinary 'import' statement.
- Enabled strict null checks in tsconfig.json
- Removed some exported classes (such as XMLDocument, HTMLDocument)
which are neither used nor exported anymore in more recent libxmljs versions.
- Added null return values to functions which sometimes return null,
e.g. prevSibling(): Node|null.