Add support for extensions / plugins

Nobody has claimed this yet.

We'd like to keep the core editor small so that it can be easily bundled on all variants of Windows. Proper syntax highlighting, language-server support, and similar, on the other hand will increase the size significantly. An extension model would allow us to split the editor up into smaller parts.

Very rough draft:
* Expose all relevant APIs (UI, text buffer) via a C ABI straight from our binary
* Add an extension loader which searches for nearby `*.dll` / `*.so` files
* Perhaps also add some certificate checks for Windows? Not sure if this exists on other platforms.
* Also, perhaps we should add some logic, so we don't load arbitrary extensions as admin / `root`
* Extensions:
* Should expose a built-with version number so we can perform compatibility checks / shims
* We then load and execute its entrypoint
* It can then load the C ABI dynamically from our executable (`GetProcAddress` / `dlsym`)
* Initially, until we stabilize the API, we should keep all important extensions in-tree in this project

[karunamurti on HN mentioned](https://news.ycombinator.com/item?id=44037422) that we could use https://github.com/bytecodealliance/wasm-micro-runtime as an alternative to loading basic dynamic libraries.

## Requested extensions

* Mun (see #53)
* Running external apps for formatting (see #136)
* A hex editor (see #152)
* An integrated terminal (see #552)
* Code formatting (see #613)

The issue names no files, tests, or entry points; start by reviewing the editor architecture and the proposed C ABI, extension loader, and wasm-micro-runtime alternative. A complete result would require an agreed extension API, loading and compatibility model, and security approach for the requested extensions.

Written by the indexing model from the issue text.