chore: Editor 'plugin' -> 'extension'

They've always been called extensions, not sure why the folder was plugins. Part of ongoing spring cleaning

shared/editor/extensions/Math.ts

@@ -0,0 +1,78 @@
+import { MathView } from "@benrbray/prosemirror-math";
+import { Node as ProseNode } from "prosemirror-model";
+import { Plugin, PluginKey, PluginSpec } from "prosemirror-state";
+import { EditorView } from "prosemirror-view";
+
+export interface IMathPluginState {
+  macros: { [cmd: string]: string };
+  activeNodeViews: MathView[];
+  prevCursorPos: number;
+}
+
+const MATH_PLUGIN_KEY = new PluginKey<IMathPluginState>("prosemirror-math");
+
+export function createMathView(displayMode: boolean) {
+  return (
+    node: ProseNode,
+    view: EditorView,
+    getPos: boolean | (() => number)
+  ): MathView => {
+    // dynamically load katex styles and fonts
+    import("katex/dist/katex.min.css");
+
+    const pluginState = MATH_PLUGIN_KEY.getState(view.state);
+    if (!pluginState) {
+      throw new Error("no math plugin!");
+    }
+    const nodeViews = pluginState.activeNodeViews;
+
+    // set up NodeView
+    const nodeView = new MathView(
+      node,
+      view,
+      getPos as () => number,
+      {
+        katexOptions: {
+          displayMode,
+          output: "html",
+          macros: pluginState.macros,
+        },
+      },
+      MATH_PLUGIN_KEY,
+      () => {
+        nodeViews.splice(nodeViews.indexOf(nodeView));
+      }
+    );
+
+    nodeViews.push(nodeView);
+    return nodeView;
+  };
+}
+
+const mathPluginSpec: PluginSpec<IMathPluginState> = {
+  key: MATH_PLUGIN_KEY,
+  state: {
+    init() {
+      return {
+        macros: {},
+        activeNodeViews: [],
+        prevCursorPos: 0,
+      };
+    },
+    apply(tr, value, oldState) {
+      return {
+        activeNodeViews: value.activeNodeViews,
+        macros: value.macros,
+        prevCursorPos: oldState.selection.from,
+      };
+    },
+  },
+  props: {
+    nodeViews: {
+      math_inline: createMathView(false),
+      math_block: createMathView(true),
+    },
+  },
+};
+
+export default new Plugin(mathPluginSpec);