Custom runners for non-Python code fences

[aaazzam]

Today, pytest-markdown-docs only collects code fences tagged py, python, or python3.

Once a fence is collected, it runs with the default runner unless the fence explicitly selects a registered custom runner, e.g.

```python runner:MyCustomRunner
```

Because collection is gated on Python fence languages, non-Python fences are ignored before custom runner selection can happen. For example, even if MyCustomRunner is registered, this fence is not collected today:

```js runner:MyCustomRunner
```

Ideally, custom runners should be able to opt non-Python fences into collection in two ways:

• runner: should explicitly opt any fence language into collection with the named registered runner.
• register_runner(default_for=(...)) should allow a runner to become the default for one or more fence languages, so users do not have to repeat runner: on every fence.

@register_runner(default_for=("js", "javascript"))
class JavascriptRunner(DefaultRunner):
    ...

With this specific design,

• A fence with runner: uses that named runner.
• A fence whose language is listed in default_for uses that language-specific default runner.
• Built-in Python fences (py, python, python3) use the global default runner.
• Other fences are ignored.