Fix expression concatenation in attribute values

- Lexer now emits a single token for the entire expression (e.g., "btn btn-" + type)
- Runtime evaluateExpression now handles + operator for string concatenation
- Added findConcatOperator to safely find operators outside quotes/brackets

src/examples/demo/views/index.pug

@@ -0,0 +1,9 @@
+doctype html
+html
+  head
+    title hello
+  body
+    p some thing
+    | ballah
+    | ballah
+    +btn("click me ", "secondary")