Moves applying default layout logic into layout.ts

For #96022

For #77131

Fixes #94516

Fixes #95846
Fixes #95851
For #77131

- Rename `EditableCustomDocument` to `CustomEditableDocument`
- Rename `CustomDocumentBackup.dispose` to `CustomDocumentBackup.delete` to make it clearer how this is supposed to be used
- Rename `OpenCustomDocumentContext` to `CustomDocumentOpenContext`
- Add a context to backup. Pass in `workspaceStorageUri`

Fix #95662

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 #95852

Fixes #95909

https://github.com/microsoft/vscode/issues/95996

fixes #95993

Fixes #95919

Fix inconsistent 'when' clause for toggle case view
Fix #95618

fixes #94285
Co-authored-by: NAlex Ross <alros@microsoft.com>

Fixes #95966

Fixes #95966