PlugOS refactor and other tweaks (#631)

* Prep for in-process plug loading (e.g. for CF workers, Deno Deploy)
* Prototype of fixed in-process loading plugs
* Fix: buttons not to scroll with content
* Better positioning of modal especially on mobile
* Move query caching outside query
* Fix annoying mouse behavior when filter box appears
* Page navigator search tweaks

server/syscalls/shell.ts

@@ -1,4 +1,3 @@
-import { shell } from "$sb/syscalls.ts";
 import { SysCallMapping } from "../../plugos/system.ts";
 import { ShellResponse } from "../../server/rpc.ts";
 import { ShellBackend } from "../shell_backend.ts";