Migrate Mathjax detection to meson

[tobiasdiez]

Moves the detection of the MathJax source file to meson.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[antonio-rojas]

Looks like there are some leftover references to the mathjax spkg

[2/190] Generating src/doc/doc-src with a custom command (wrapped by meson to set env)
Traceback (most recent call last):
  File "/build/sagemath-doc-git/src/sage/build/bin/sage-package", line 40, in <module>
    run()
    ~~~^^
  File "/build/sagemath-doc-git/src/sage/build/bin/../sage_bootstrap/cmdline.py", line 520, in run
    app.dependencies(*args.package_class, types=types, format=args.format)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/build/sagemath-doc-git/src/sage/build/bin/../sage_bootstrap/app.py", line 183, in dependencies
    elif format == 'rst' and Package(dep).has_file('SPKG.rst'):
                             ~~~~~~~^^^^^
  File "/build/sagemath-doc-git/src/sage/build/bin/../sage_bootstrap/package.py", line 53, in __new__
    self.__init__(package_name)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/build/sagemath-doc-git/src/sage/build/bin/../sage_bootstrap/package.py", line 85, in __init__
    self._init_type()
    ~~~~~~~~~~~~~~~^^
  File "/build/sagemath-doc-git/src/sage/build/bin/../sage_bootstrap/package.py", line 550, in _init_type
    with open(os.path.join(self.path, 'type')) as f:
         ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/build/sagemath-doc-git/src/sage/build/pkgs/mathjax/type'