fix: resolve conditional return string branch for interpolated strings

The AST-based conditional return resolver only matched
Expression::Literal(Literal::String) when checking `$key is string`,
so interpolated strings like `"{$prefix}.host"` (parsed as
Expression::CompositeString) fell through to the else branch and
resolved as null. Add CompositeString to the string match so the
correct branch is taken.

Closes #269

Author: calebdw

tests/integration/hover.rs

@@ -1,7 +1,7 @@
 //! Integration tests for `textDocument/hover`.
 
 use crate::common::{
-    create_psr4_workspace, create_test_backend, create_test_backend_with_closure_stub,
+    create_psr4_workspace create_test_backend, create_test_backend_with_closure_stub,
     create_test_backend_with_full_stubs, create_test_backend_with_function_stubs,
     create_test_backend_with_stdclass_stub,
 };