Fix parsing for imports using non-alphanumeric assignments

I noticed a bug in the brigade codebase where import parsing had stopped
at this line:

const $ = require('jquery');

Which is a perfectly valid import. I had been a little bit too
aggressive in 73bce8a when I fixed a different parsing bug.

lib/import_js/import_statement.rb

@@ -75,7 +75,7 @@ def self.parse(string)
         named_imports = dest_match[:named].split(/,\s*/).map(&:strip)
       else
         default_import = match[:assignment]
-        return unless default_import =~ /\A\w+\Z/
+        return unless default_import =~ /\A[^\s]+\Z/
       end
 
       new(

spec/import_js/import_statement_spec.rb

@@ -13,6 +13,15 @@
         expect(subject.path).to eq('foo')
       end
 
+      context 'and it has a non-alphanumeric variable name' do
+        let(:string) { "import $ from 'jquery';" }
+
+        it 'returns a valid ImportStatement instance' do
+          expect(subject.assignment).to eq('$')
+          expect(subject.path).to eq('jquery')
+        end
+      end
+
       context 'and it has line breaks' do
         let(:string) { "import foo\n  from 'foo';" }