Make the Relay Compiler Terminal Transient (#4455)

Summary:
If you enable `relay.autoStartCompiler`, the extension will open a compiler in a new Terminal on every VS Code reload (e.g. Developer: Reload Window). This can easily lead to many copies of the compiler running.

Instead, we can mark the terminal with `isTransient`, and it won't survive reloads. While this means reloads must restart the compiler, it is very fast and this avoid many "Relay Compiler" terminals.

Pull Request resolved: #4455

Test Plan:
- Debug on a project with `"relay.autoStartCompiler": true`
- Reload the window
- Note there is only one "Relay Compiler" terminal

Reviewed By: alunyov

Differential Revision: D50021664

Pulled By: captbaritone

fbshipit-source-id: 8f725eff1d5e76d98e689641ee64c6a81b587a06

vscode-extension/src/compiler.ts

@@ -25,6 +25,7 @@ export function createAndStartCompiler(context: RelayExtensionContext) {
   const terminal = window.createTerminal({
     name: 'Relay Compiler',
     cwd: context.relayBinaryExecutionOptions.rootPath,
+    isTransient: true,
   });
 
   terminal.sendText(