Add a context menu for the shader editor file list

[SomeRanDev]

I frequently right-click on items in the shader editor expecting a context menu like the script editor, and it's very jarring when nothing happens. This adds a basic context menu with the same structure as the one from the script editor; it includes a couple new options like "Close Other Tabs", "Close All", and "Show in File System."

Relevant issue: https://github.com/godotengine/godot/issues/75061

[jsjtxietian]

I think this can be merged along with https://github.com/godotengine/godot/pull/84063, to make the shader editor's usability better

[SomeRanDev]

Noted! 👌

I'll also do a PR for "Sort". It was originally going to be part of this, but I decided to stick with the bare essentials for the initial context menu.

[akien-mga]

Needs rebase (probably due to #84063).

[SomeRanDev]

Rebased and ready to go (after checks)! 👍

[akien-mga]

Thanks!