webview: respect default localResourceRoots for custom editors

Custom-editor flow constructs the webview overlay on the main thread
with empty contentOptions and never propagates the documented default
localResourceRoots (workspace folders + extension install directory),
so asWebviewUri returns 401 unless the extension explicitly sets
webview.options. The regular createWebviewPanel flow already applies
this default in ExtHostWebviewPanels.createWebviewPanel.

Add ExtHostWebviews.ensureDefaultContentOptions which calls $setOptions
with the documented default when localResourceRoots is absent, and
invoke it from $resolveCustomEditor right after createNewWebview.

Fixes #282495