Implement basic execution system and UI

This is a rather large commit that includes all of the following:
- React UI with code editor, runtime renderer and input-output panes
- Language providers for a sample language and Brainfuck
- Implementation of code execution in a web worker
- All-at-once unabortable execution of program fully functional

ui/output-viewer.tsx

@@ -0,0 +1,33 @@
+import { TextArea } from "@blueprintjs/core";
+
+/**
+ * For aesthetic reasons, we use readonly textarea for displaying output.
+ * Textarea displays placeholder if value passed is empty string, which is undesired.
+ * This function is a fake-whitespace workaround.
+ *
+ * @param value Value received from parent. Placeholder shown on `null`.
+ * @returns Value to pass as prop to Blueprint TextArea
+ */
+const toTextareaValue = (value: string | null): string | undefined => {
+  if (value == null) return undefined; // Placeholder shown
+  if (value === "") return "\u0020"; // Fake whitespace to hide placeholder
+  return value; // Non-empty output value
+};
+
+type Props = {
+  value: string | null;
+};
+
+export const OutputViewer = ({ value }: Props) => {
+  return (
+    <TextArea
+      fill
+      large
+      readOnly
+      growVertically
+      value={toTextareaValue(value)}
+      placeholder="Run code to see output..."
+      style={{ height: "100%", resize: "none", boxShadow: "none" }}
+    />
+  );
+};