Add set_environment() method in OS

[YakoYakoYokuYoku]

Describe the project you are working on:
A Cargo plugin for the editor.

Describe the problem or limitation you are having in your project:
Trying to set an environment variable in Godot without using the system shell is impossible, something that might not be desired in some cases.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Adding a void set_evironment(String env_var, String value) method to the OS class provides a simpler and cleaner way to set the variables.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
As environment variables have strings as their values the set_environment() method must take the variable (env_var) and the value (value). Then assigns value to env_var. E.g.:

OS.set_environment("FOO", "bar")

If this enhancement will not be used often, can it be worked around with a few lines of script?:

OS.execute(<"export" for UNIX, "set" for Windows>, ["FOO=bar"])

Is there a reason why this should be core and not an add-on in the asset library?:
Considering many programming languages provides the method/functionality in their standard libraries and Godot has the get_environment() method, this proposal brings a solution to the problem without any workarounds.

[YakoYakoYokuYoku]

@Calinou this was accepted in PR godotengine/godot/pull/46413.