Set session socket into environment variable (#6282)

* Avoid spawning code-server with --reuse-window and --new-window

These flags mean the user explicitly wants to open in an existing
instance so if the socket is down it should error rather than try to
spawn code-server normally.

* Set session socket into environment variable

While I was at it I added a CLI flag to override the default.  I also
swapped the default to --user-data-dir.

The value is set on an environment variable so it can be used by the
extension host similar to VSCODE_IPC_HOOK_CLI.

* Add e2e test for opening files externally

test/unit/node/vscodeSocket.test.ts

@@ -1,19 +1,8 @@
 import { logger } from "@coder/logger"
 import * as app from "../../../src/node/app"
-import { paths } from "../../../src/node/util"
-import {
-  DEFAULT_SOCKET_PATH,
-  EditorSessionManager,
-  makeEditorSessionManagerServer,
-} from "../../../src/node/vscodeSocket"
+import { EditorSessionManager, makeEditorSessionManagerServer } from "../../../src/node/vscodeSocket"
 import { clean, tmpdir, listenOn, mockLogger } from "../../utils/helpers"
 
-describe("DEFAULT_SOCKET_PATH", () => {
-  it("should be a unique path per user", () => {
-    expect(DEFAULT_SOCKET_PATH.startsWith(paths.data)).toBe(true)
-  })
-})
-
 describe("makeEditorSessionManagerServer", () => {
   let tmpDirPath: string