(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/StringUnion.ts

@@ -0,0 +1,38 @@
+/**
+ * TypeScript will infer a string union type from the literal values passed to
+ * this function. Without `extends string`, it would instead generalize them
+ * to the common string type.
+ *
+ * Example definition:
+ * const Race = StringUnion(
+ *   "orc",
+ *   "human",
+ *   "night elf",
+ *   "undead",
+ * );
+ * type Race = typeof Race.type;
+ *
+ * For more details, see:
+ * https://stackoverflow.com/questions/36836011/checking-validity-of-string
+ *   -literal-union-type-at-runtime?answertab=active#tab-top
+ */
+export const StringUnion = <UnionType extends string>(...values: UnionType[]) => {
+  Object.freeze(values);
+  const valueSet: Set<string> = new Set(values);
+
+  const guard = (value: string): value is UnionType => {
+    return valueSet.has(value);
+  };
+
+  const check = (value: string): UnionType => {
+    if (!guard(value)) {
+      const actual = JSON.stringify(value);
+      const expected = values.map(s => JSON.stringify(s)).join(' | ');
+      throw new TypeError(`Value '${actual}' is not assignable to type '${expected}'.`);
+    }
+    return value;
+  };
+
+  const unionNamespace = {guard, check, values};
+  return Object.freeze(unionNamespace as typeof unionNamespace & {type: UnionType});
+};