Includes tests around API stability, as these expectations were all broken in a previous naive attempt at the same change

1. Introduces an abstract subset of DOM APIs corresponding to previous usage of semantically equivalent `libxmljs` APIs. It would have been nicer to import these types and export the subset we care about, but this is inconsistent with how TypeScript `lib` works (it always augments the global scope).

2. Provides a Node implementation of those DOM APIs by extending `libxmljs` prototypes. This kind of extension isn't ideal, but it's the most reasonable way to achieve such a compatibility layer without sacrificing performance. An ealirer alternative approach used `WeakMap`s to cross reference the DOM/`libxmljs` interfaces, but this had a significant impact on perf. This implementation is type checked by default.

3. Provides a web implementation by... just exporting the relevant globals. This implementation is type checked by `tsc` using the `tsconfig.web.json` project file. This ensures that the abstract DOM interfaces are actully consistent with the built in DOM `lib` types.

4. Refactors transformer.ts to use the abstract DOM APIs. This uses the Node DOM compatibility implementation by default, and the native web DOM implementation when the environment variable `ENV` is set to "web".

5. Also when `ENV` is web, tests and benchmarks call `transform` in the specified `BROWSER` environment variable (defaulting to "fiefox") via a simple `playwright` bridge. In CI, all tests and benchmarks are run in: Node 14, Node 16, Firefox, Chromium, Webkit.

6. Build is updated to produce both Node and web targets. The build config itself is fairly complex, but it's been consolidated in `vite.config.ts`. This also includes a corresponding change to build `app.ts` rather than the previous, much more complex `app.js` using a Vite dev server (and roughly restores its implementation to what it had been prior to the initial TypeScript migration).

7. There are several known issues at the point of this commit. These either correspond to XSLT extensions not supported by browser targets, or to differences in behavior between DOM/`libxmljs`, all of which will be addressed in separate commits discussing each in greater detail.

This addresses a slight difference between Node's and browsers' respective implementations of `URL`, namely that browsers do not implicitly escape special path characters for unknown schemes (i.e. `jr`). This change is a workaround, temporarily substituting the `jr` scheme to consistently take advantage of built-in escaping behavior, then restoring it for the final return value.

This addresses:

- an inconsistency between how `libxslt` and the native DOM `XSLTProcessor` handle XSLT transforms to HTML.`libxslt` (correctly, IMO) treats the XSLT document's expressed root element as the resulting document element, whereas browsers produce a document with an implicit `html > body > root` structure.

- a slight change in `renderMarkdown` which parses to a document rather than a fragment, for the purpose of a simpler DOM compatibiity API in this case. (That call to `correctHTMLDocHierarchy` was mistakenly included in an earlier commit, but I'm hoping this subsequent commit will be clear enough and we'll likely squash these commits anyhow.)

The functionality depending on them will be reimplemented with DOM APIs

It's possible there are other attribute cases I haven't identified, but these are the ones I was able to find in test failures, snapshot mismatches, and the spec.

These dynamic template calls are now injected into the XSL, allowing us to eliminate the use of `str:replace` and `dyn:evaluate` extensions.

This eliminates the need to use `str:tokenize` extension

Faster Firefox startup, improved logging

Also bumps actions versions which have been warning about Node 12 for no apparent reason

- Update README
- Clarify change to URL escaping implementation per initial review feedback

…lates into XSLT

- Add demo dev mode (very useful for manual testing during dev)
- Actually display errors
- Don't show Invalid state on errors