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/e2e/terminal.test.ts

@@ -35,13 +35,19 @@ describe("Integrated Terminal", ["--disable-workspace-trust"], {}, () => {
     const tmpFolderPath = await tmpdir(testName)
     const tmpFile = path.join(tmpFolderPath, "test-file")
     await fs.writeFile(tmpFile, "test")
-    const fileName = path.basename(tmpFile)
 
     await codeServerPage.focusTerminal()
 
     await codeServerPage.page.keyboard.type(`code-server ${tmpFile}`)
     await codeServerPage.page.keyboard.press("Enter")
 
-    await codeServerPage.waitForTab(fileName)
+    await codeServerPage.waitForTab(path.basename(tmpFile))
+
+    const externalTmpFile = path.join(tmpFolderPath, "test-external-file")
+    await fs.writeFile(externalTmpFile, "foobar")
+
+    await codeServerPage.openFileExternally(externalTmpFile)
+
+    await codeServerPage.waitForTab(path.basename(externalTmpFile))
   })
 })