Let an extension own a file type

[Seungpyo1007]

The remaining half of track C-3 from the 0.3.0 plan.

An extension can own a file type

contributes.customEditors declares which patterns a viewType claims; registerCustomEditorProvider supplies the implementation. Both have to be present — a declaration alone falls back to the text editor, which is better than a blank pane when the extension has not activated yet. That pairing is src/ext/customEditors.ts, pure, with 18 tests.

The document stays a normal TextDocument, so an extension edits through WorkspaceEdit and the change goes via the model — Ctrl+Z still works and the save baseline stays honest.

CustomEditorProvider — the binary kind that owns its own bytes, save and backup — is still not supported, and registering one throws at registration naming what is missing. Registering it silently would open that file type to a blank pane.

Two defects found while building it

• .draw files have no model. preload only creates models for TS-like files, and a file claimed by a custom editor never gets a Monaco pane either, so nothing created one. The extension received null and died on its first document.getText(). The document is now read and created on demand.
• The iframe had no acquireVsCodeApi. The extension's own script died on its first line and the webview could not talk back at all. It now goes through the same bridge the sidebar webviews use.

README corrected

It claimed the shim has no custom editors and that WorkspaceEdit cannot create, delete or rename files — the latter has been false since #71, and debug observation since #79. Rewritten to what is actually true, with the two genuinely missing pieces named.

Verification

Real app with a probe extension owning *.draw, 5/5: the file opens in the extension's iframe rather than Monaco, resolveCustomTextEditor is called, the document text reaches the extension ("SHAPES: circle, square\n"), a message from the webview arrives at the extension, and a file it does not claim still opens as text.

1133 unit tests passing (18 new). npm run typecheck and npm run build clean.