Support for text encodings other than UTF-8 and ASCII in the text preview handler

### Description of the new feature / enhancement

Allow us to override the current text encoding with a simple dropdown control in the text preview pane context menu.

### Scenario when this would be used?

In every text preview where the handler visibly failed to recognize the text encoding (if does any recognition at all).

### Supporting information

UTF-16 LE/BE are not supported and likely others as well. UTF-16 LE text files are shown with a bunch of null chars in the preview pane, rendering them (almost) unreadable.

In VS Code:

In Preview Pane:

Start at the text preview handler and its pane context menu, then inspect how UTF-8 and ASCII are currently decoded. Add a simple encoding override control, including UTF-16 LE/BE support, and verify that affected text previews render without null characters.

Written by the indexing model from the issue text.