Add JIT utilities in `sys._jit`

[brandtbucher]

Feature or enhancement

...as discussed in https://discuss.python.org/t/an-api-for-controlling-and-introspecting-the-jit/73356. This will include (for 3.14):

• sys._jit.is_available() -> bool: Return True if the current Python executable supports JIT compilation, and False otherwise.
• sys._jit.is_enabled() -> bool: Return True if JIT compilation is enabled for the current Python process (implies sys._jit.is_available()), and False otherwise.
• sys._jit.is_active() -> bool: Return True if the topmost Python frame is currently executing JIT code, and False otherwise.

Linked PRs

• GH-133231: Add JIT utilities in sys._jit #133233
• GH-133231: Changes to executor management to support proposed sys._jit module #133287
• [3.13] GH-133231: Backport PYTHON_JIT documentation to 3.13 #133539