[3.x] Allow reading shaders from .gdshader files

[aaronfranke]

This PR adds support for reading .gdshader files, which adds forwards compatibility with #47336.

Shaders seem to be the same between 3.x and the current master (at least the code for simple ones hasn't changed), it's only the file extension that changed. Since only .gdshader can be read in 4.0, once that comes out, shader authors will likely use .gdshader. If we allow 3.x to read these files, then these shaders can be easily used in 3.x without renaming the files. This will also make it easier for people to future-proof their projects for eventual conversion to Godot 4.0 (at the cost of the shaders not being able to be read in Godot 3.3.2 or lower). But for now I would still recommend users use .shader (or .tres).

When saving, the default file extension is .tres (same as the current 3.x), and both .shader and .gdshader are options.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.3.3.