ts_project_worker: default-initialize tsc outDir

Some time ago, the tsc_project macro changed behavior.
Before, `out_dir` would always be set, whereas now it may be `None`.
If `out_dir` is unset, this expression will result in an incorrect
output directory being used:

```
host.optionsToExtend.outDir = `${host.optionsToExtend.outDir}/${SYNTHETIC_OUTDIR}`;
```

(Would result in `'undefined/__st_outdir__'`).

Author: malt3

ts/private/ts_project_worker.js

@@ -704,7 +704,7 @@ function createProgram(args, inputs, output, exit) {
     }
 
     function applySyntheticOutPaths() {
-        host.optionsToExtend.outDir = `${host.optionsToExtend.outDir}/${SYNTHETIC_OUTDIR}`;
+        host.optionsToExtend.outDir = `${host.optionsToExtend.outDir || host.optionsToExtend.rootDir || '.'}/${SYNTHETIC_OUTDIR}`;
         if (host.optionsToExtend.declarationDir) {
             host.optionsToExtend.declarationDir = `${host.optionsToExtend.declarationDir}/${SYNTHETIC_OUTDIR}`
         }