Support sagemath language

[Eloitor]

Hi this is my first contribution to this project. I copied the structure from: #2903

The goal of this PR is to add support for the sagemath language.

• The syntax is based on python.
• I didn't succeed at running the python language server by default with .sage files.
• It would be nice if the syntax highlighting supported sage's docstrings: In a comment, the lines starting with sage: contain sage code. It would be nice if its syntax were highlighted. For an example of a docstring, see: https://doc.sagemath.org/html/en/developer/coding_basics.html#template

[the-mikedavis]

Could you run cargo xtask docgen and commit the results? That will add sage to the supported language documentation