ENH Support SDL-based packages and add pyxel. ### Description

This adds SDL-based package support to Pyodide and adds pyxel as a PoC. Most of the troubleshooting is done by @kitao and pyxel devs, a big thanks to them.

This is on top of #3505

1) I linked only js part of SDL libraries to the main module and linked the native parts to the side module. So the size increase of the main module is small enough (~0.1MB increase to the pyodide.asm.js).

Before (in #3505):

```
5145705 pyodide.asm.data
1204075 pyodide.asm.js
8862216 pyodide.asm.wasm
```

After:

```
5145705 pyodide.asm.data
1346524 pyodide.asm.js
8905514 pyodide.asm.wasm
```

2) Emscripten draws everything into `Module.canvas`, but the canvas attribute is not set automatically (which makes sense as we don't output any HTML), so it is the user's job to set it manually.

For now, I am thinking of just mentioning it in the FAQ entry, "you need to set pyodide._module.canvas = ...". But it would be probably nice to have an API to set canvas, as pyodide._module is a private attribute.

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

### Demo

https://ryanking13.github.io/pyodide-pyxel-test

(mostly borrowed from the pyxel repository: https://github.com/kitao/pyxel/tree/main/wasm)

// The point is to make a file that works with JavaScript analysis tools like
// JsDoc and LGTM. They want to parse the file as JavaScript. Thus, it's key
// that included js files should parse as valid JavaScript. `JS_FILE` is a
// specially designed macro to allow us to do this. We need to look like a
// function call to JavaScript parsers. The easiest way to get it to parse is to
// make the macro argument look like a JavaScript anonymous function, which we
// do with `()=>{`. However, `()=>{` is an invalid C string so the macro needs
// to remove it. We put `()=>{0,0;`, JS_FILE removes everything up to
// the comma and replace it with a single open brace.
//

#define UNPAIRED_OPEN_BRACE {
#define UNPAIRED_CLOSE_BRACE } // Just here to help text editors pair braces up
#define JS_FILE(func_name, a, args...)                                         \
  EM_JS_NUM(                                                                   \
    int, func_name, (), UNPAIRED_OPEN_BRACE { args return 0; })

// A macro to allow us to add code that is only intended to influence JsDoc
// output, but shouldn't end up in generated code.
#define FOR_JSDOC_ONLY(x)

// The point is to make a file that works with JavaScript analysis tools like
// JsDoc and LGTM. They want to parse the file as JavaScript. Thus, it's key
// that included js files should parse as valid JavaScript. `JS_FILE` is a
// specially designed macro to allow us to do this. We need to look like a
// function call to JavaScript parsers. The easiest way to get it to parse is to
// make the macro argument look like a JavaScript anonymous function, which we
// do with `()=>{`. However, `()=>{` is an invalid C string so the macro needs
// to remove it. We put `()=>{0,0;`, JS_FILE removes everything up to
// the comma and replace it with a single open brace.
//

#define UNPAIRED_OPEN_BRACE {
#define UNPAIRED_CLOSE_BRACE } // Just here to help text editors pair braces up
#define JS_FILE(func_name, a, args...)                                         \
  EM_JS_NUM(                                                                   \
    int, func_name, (), UNPAIRED_OPEN_BRACE { args return 0; })

// A macro to allow us to add code that is only intended to influence JsDoc
// output, but shouldn't end up in generated code.
#define FOR_JSDOC_ONLY(x)
