[red-knot] Enforce specifying paths for mdtest code blocks in a separate preceding line (#15890)

## Summary

Resolves #15695, rework of #15704.

This change modifies the Mdtests framework so that:

* Paths must now be specified in a separate preceding line:

	`````markdown
	`a.py`:

	```py
	x = 1
	```
	`````

If the path of a file conflicts with its `lang`, an error will be
thrown.

* Configs are no longer accepted. The pattern still take them into
account, however, to avoid "Unterminated code block" errors.
* Unnamed files are now assigned unique, `lang`-respecting paths
automatically.

Additionally, all legacy usages have been updated.

## Test Plan

Unit tests and Markdown tests.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/shadowing/function.md

@@ -5,14 +5,18 @@
 Parameter `x` of type `str` is shadowed and reassigned with a new `int` value inside the function.
 No diagnostics should be generated.
 
-```py path=a.py
+`a.py`:
+
+```py
 def f(x: str):
     x: int = int(x)
 ```
 
 ## Implicit error
 
-```py path=a.py
+`a.py`:
+
+```py
 def f(): ...
 
 f = 1  # error: "Implicit shadowing of function `f`; annotate to make it explicit if this is intentional"
@@ -20,7 +24,9 @@ f = 1  # error: "Implicit shadowing of function `f`; annotate to make it explici
 
 ## Explicit shadowing
 
-```py path=a.py
+`a.py`:
+
+```py
 def f(): ...
 
 f: int = 1