chore: import upstream snapshot with attribution

js/code/shared/extensions.ts

@@ -0,0 +1,47 @@
+import type { Extension } from "@codemirror/state";
+import {
+	lineNumbers,
+	highlightSpecialChars,
+	drawSelection,
+	rectangularSelection,
+	crosshairCursor,
+	keymap
+} from "@codemirror/view";
+export { EditorView } from "@codemirror/view";
+import { EditorState } from "@codemirror/state";
+import {
+	foldGutter,
+	indentOnInput,
+	syntaxHighlighting,
+	defaultHighlightStyle,
+	foldKeymap
+} from "@codemirror/language";
+import { history, defaultKeymap, historyKeymap } from "@codemirror/commands";
+import {
+	closeBrackets,
+	closeBracketsKeymap,
+	completionKeymap
+} from "@codemirror/autocomplete";
+import { lintKeymap } from "@codemirror/lint";
+
+export const basicSetup: Extension = /*@__PURE__*/ ((): Extension[] => [
+	highlightSpecialChars(),
+	history(),
+	foldGutter(),
+	drawSelection(),
+	EditorState.allowMultipleSelections.of(true),
+	indentOnInput(),
+	syntaxHighlighting(defaultHighlightStyle, { fallback: true }),
+	closeBrackets(),
+	rectangularSelection(),
+	crosshairCursor(),
+
+	keymap.of([
+		...closeBracketsKeymap,
+		...defaultKeymap,
+		...historyKeymap,
+		...foldKeymap,
+		...completionKeymap,
+		...lintKeymap
+	])
+])();