Possibly fixes build stalling issue (#90757)

## About The Pull Request
Per the discord: rspack has been randomly freezing during build. This is
possibly related to https://github.com/web-infra-dev/rspack/issues/8291

I'm not certain this will fix it, but it's worth giving it a try as it
doesn't seem to change anything otherwise in local builds
## Why It's Good For The Game
TGS works
## Changelog

tgui/rspack.config.cjs

@@ -78,6 +78,7 @@ module.exports = (env = {}, argv) => {
             {
               loader: 'builtin:swc-loader',
               options: {
+                isModule: 'unknown',
                 jsc: {
                   parser: {
                     syntax: 'typescript',