Fix Babel transpile for EXPORT_ES6 by using sourceType module

When EXPORT_ES6 is enabled, makeNodeImport() generates top-level
await import() expressions. Babel's transpile step was configured
with sourceType 'script', which cannot parse top-level await.
This caused test_node_prefix_transpile to fail at compilation.

Use sourceType 'module' when EXPORT_ES6 is active so Babel can
correctly parse and transpile the ESM output.

Author: Bartek Wrona

tools/building.py

@@ -541,7 +541,7 @@ def version_split(v):
 @ToolchainProfiler.profile()
 def transpile(filename):
   config = {
-    'sourceType': 'script',
+    'sourceType': 'module' if settings.EXPORT_ES6 else 'script',
     'presets': ['@babel/preset-env'],
     'plugins': [],
     'targets': {},