Add Pip Requirements language

[spenserblack]

This language is a text file that lists requirements for the pip install command, most commonly a list of packages with optional version constraints.

Checklist:

• I am adding a new language.
  • The extension filename of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3Arequirements.txt+Django
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3Arequirements-dev.txt+black
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • requirements-dev.txt
      • requirements.txt
    • Sample license(s):
      • requests
      • stable-diffusion
  • I have included a syntax highlighting grammar: https://github.com/Microsoft/vscode-python
  • I have added a color
    • Hex value: #FFD343
    • Rationale: This color was taken from the Python website (https://www.python.org/), picking out the (seemingly) primary yellow color. The logo is made up of two colors: blue and yellow. With blue being the color associated with Python, yellow was chosen as a counterpart for the language's package installer's requirements file.
  • I have updated the heuristics to distinguish my language from others using the same extension.

[spenserblack]

I wasn't sure if heuristics are needed because, while these files do conflict with the .txt extension, this language only adds filenames, which as I understand take higher priority over extensions.

There might need to be some discussion on the language's name -- according to the docs, another name is "pip requirements.txt files".

[lildude]

LGTM and there shouldn't be any conflicts as explicit names take priority. I think the name is a good start.