Allow .css file to be imported and compiled like an .styl file

[yuriuseu]

Discussed in #2739

^{Originally posted by Yuriuseu November 25, 2022}
Say, I want to declare variables in plain.css file:

variable = "value"

and import that in another.css file:

@import "plain.css";

.class {
  property: variable; // variable is treated as a literal value of property
}

This compiles as intended if we declare the variable directly or if we change the extension of plain.css to .styl.

Now, I understand that the syntax will be messy in some editors, but considering that a single .css file with stylus content can be compiled normally in the first place, so why not allow plain .css to be treated just like an .styl file when imported?