(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/server/lib/docUtils.d.ts

@@ -0,0 +1,9 @@
+export function makeIdentifier(name: string): string;
+export function copyFile(src: string, dest: string): Promise<void>;
+export function createNumbered(name: string, separator: string, creator: (path: string) => Promise<void>,
+                               startNum?: number): Promise<string>;
+export function createNumberedTemplate(template: string, creator: (path: string) => Promise<void>): Promise<string>;
+export function createExclusive(path: string): Promise<void>;
+export function realPath(path: string): Promise<string>;
+export function pathExists(path: string): Promise<boolean>;
+export function isSameFile(path1: string, path2: string): Promise<boolean>;