This change primarly adds a new `Open with...` entry to the explorer context menu. To do this however, I had to make a few other changes:

- Add a new explorer context key for availible editors
- Moved the editor select prompt into a new function called `openEditorWith`
- Use `openEditorWith` for the new `open with` explorer command as well as for the `reopen with` command

For #94408

Moves this out of custom editors

So far, file editors have reported isDirty() === false when auto save was enabled e.g. to avoid showing dirty indicators. With the advent of custom editors, this needs to be cleaned up. The new method isSaving() serves as a hint that an editor can be dirty but in the process of being saved. As such, we can decide to hide the dirty indicator in that case. But in other cases we may want to know if the editor is dirty or not, even if auto saved.

Fix #87487

#48258

supporting viewpanecontainer in panels with PaneCompositePanel and ViewContainerLocation

fixes #48258

Part of microsoft/vscode-remote-release#1777

- Import aliasing symbol in file
- Missing function calls in conditional
- Missing return statement

fixes #76752

* fix #72924

* add test for FileOnDiskContentProvider

* make test assert more