Disallow exports that are not valid C/C++ identifiers

I use `std.isdentifier()` here from python since it has the same rules
for valid identifiers are C/C++.

We could try instead to allow this but I'm not sure it worth it given
that our inputs are almost exclusively compiled from C/C++/rust.

See #23560

test/test_other.py

@@ -15539,3 +15539,8 @@ def test_cxx20_modules_std_headers(self):
     }
     ''')
     self.do_runf('main.cpp', 'Hello Module!', emcc_args=['-std=c++20', '-fmodules'])
+
+  def test_invalid_export_name(self):
+    create_file('test.c', '__attribute__((export_name("my.func"))) void myfunc() {}')
+    err = self.expect_fail([EMCC, 'test.c'])
+    self.assertContained('emcc: error: invalid export name: my.func', err)

tools/emscripten.py

@@ -578,6 +578,9 @@ def finalize_wasm(infile, outfile, js_syms):
   # These are any exports that were not requested on the command line and are
   # not known auto-generated system functions.
   unexpected_exports = [e for e in metadata.all_exports if treat_as_user_export(e)]
+  for n in unexpected_exports:
+    if not n.isidentifier():
+      exit_with_error(f'invalid export name: {n}')
   unexpected_exports = [asmjs_mangle(e) for e in unexpected_exports]
   unexpected_exports = [e for e in unexpected_exports if e not in expected_exports]