Add "Inspect Native Shader Code" to shader inspector and shader editor

[tetrapod00]

Implements and closes godotengine/godot-proposals#10278.

Adds the action "Inspect Native Shader Code", currently available on the Material resource inspector (implementation), to the Shader resource inspector and to the shader editor.

I added the action to the shader editor under the File menu, because it's a valid action for both text and visual shaders. I can see reasons to instead do one of the following:

• Add the action to one of the other menus, like Edit, which only appear for text shaders. This action is for advanced users, currently takes multiple seconds to open on my machine, and may be confused with the "Show Generated Shader Code" action in visual shaders. On the other hand, it is a valid action for both types of shader.
• Not include this action in the shader editor at all. Perhaps it will cause more confusion than its worth. With this PR, you can now do File -> Open File in Inspector and then Manage object properties -> Inspect Native Shader Code.