Back-to-back math environments are parsed incorrectly

[masonproffitt]

For example:

>>> import TexSoup
>>> TexSoup.TexSoup(r'$1$$2$')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/__init__.py", line 87, in TexSoup
    parsed, src = read(tex_code, skip_envs=skip_envs, tolerance=tolerance)
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/tex.py", line 22, in read
    return TexEnv('[tex]', begin='', end='', contents=buf), tex
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/data.py", line 892, in __init__
    super().__init__(name, contents, args, preserve_whitespace, position)
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/data.py", line 641, in __init__
    self._contents = list(contents) or []
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 52, in read_tex
    yield read_expr(buf,
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 96, in read_expr
    return read_math_env(src, expr, tolerance=tolerance)
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 213, in read_math_env
    contents.append(read_expr(src, tolerance=tolerance, mode=MODE_MATH))
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 96, in read_expr
    return read_math_env(src, expr, tolerance=tolerance)
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 213, in read_math_env
    contents.append(read_expr(src, tolerance=tolerance, mode=MODE_MATH))
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 96, in read_expr
    return read_math_env(src, expr, tolerance=tolerance)
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 215, in read_math_env
    unclosed_env_handler(src, expr, src.peek())
  File "/home/user/atlas/physics-office/src/TexSoup/TexSoup/reader.py", line 189, in unclosed_env_handler
    raise EOFError('[Line: %d, Offset: %d] "%s" env expecting %s. %s' % (
EOFError: [Line: 0, Offset: 5] "$" env expecting $. Reached end of file.

[JCGoran]

I've narrowed down the issue to this piece of code:
https://github.com/alvinwan/TexSoup/blob/c91a14a0019ff7df197e71c906bc0403eddf80dc/TexSoup/tokens.py#L176C1-L181

The problem is that it checks for $$ first, but it isn't aware that it's already in (single-line) math mode, so it erroneously thinks it's entering display math mode instead. Not exactly sure how to fix this since it requires knowledge of state, which isn't passed to the function currently.