Fix undo/redo broken in non-editor webviews

[evangrayk]

This PR fixes #89960

Undo/redo is broken in webviews due to the context key being overly aggressive in setting itself as webviewHasOwnEditFunctions. I think instead of true, this should set it to be based on if it has a custom editor. This fixes webviewCommands.ts, which uses this context key for undo/redo.

Repro:

1. open a webview with an <input> (e.g. modify the cat coding sample)
2. type into the input
3. undo & redo with cmd-z and cmd-shift-z work as expected

[mjbvz]

Thanks!