Mammoth

* Add style mapping for highlights.

* Remove the use of the path module from common code used by browser builds.
This should remove the need to polyfill path in the browser.

* Handle numbering level definitions without an explicit format.

* Switch the precedence of numbering properties in paragraph properties and the
numbering in paragraph styles so that the numbering properties in paragraph
properties takes precedence.

* Support attributes in HTML paths in style mappings.

* Improve error message when failing to find the body element in a document.

* Add support for the strict document format.

* Add transformDocument to the TypeScript declarations.

* Support merged paragraphs when revisions are tracked.

* Use xmldom instead of sax to parse XML documents. This should remove the need
to polyfill stream in the browser.

* Adjust the internal implementation to remove the use of Buffer on the critical
path, and provide APIs to read images and documents with embedded style maps
without using Buffer. This should remove the need to polyfill Buffer in the
browser. Since TextDecoder is now used, the minimum version of node.js is now
v12.

* Remove the use of the util module. This should remove the need to polyfill
util in the browser.

* Fix: npm 7 changed the behaviour of prepublish, causing the browser build not
to be updated before publishing to npm. We now use prepare instead of
prepublish, which has the same behaviour that prepublish previously had.