Simple file dialog: allow editing folder name on create-folder prompt

[Copilot]

In the simple file dialog's "The folder X does not exist. Would you like to create it?" prompt, any keystroke would invisibly dismiss the prompt (via onDidChangeValue → hide()), making it look like typing "did nothing".

Changes

• yesNoPrompt: added allowEdit flag; return type changed from Promise<boolean> to Promise<URI | undefined> so callers can read back the (possibly edited) path. With allowEdit=true the auto-dismiss-on-change handler is not registered, and on cancel-after-edit the typed value is propagated back to filePickBox.value so edits aren't lost.
• Open / create-folder path: invokes yesNoPrompt(uri, message, true). If the accepted URI differs from the original, the new value is pushed into the file picker and validation re-runs (re-prompting against the new name) rather than creating a folder at the edited path while the outer onDidAccept still resolves with the stale resolveValue.
• Save-mode prompts (overwrite / create save destination): kept the previous cancel-on-edit semantics — adapted to the new signature via !!(await this.yesNoPrompt(...)). These flows have no path to re-validate an edited URI before saving, so silently changing them would risk saving at the wrong location.