Implement ? (question mark) operator for error propagation

The ? operator now desugars expr? to a match expression that unwraps Ok
values and early returns Err values:

  match expr {
      Ok(#ok) => #ok,
      Err(#err) => return Err(#err),
  }

Implementation details:
- Use pre-interned identifiers (#ok, #err) for synthetic variables
- Use pre-interned Ok/Err tag names from CommonIdents
- Create applied_tag patterns for Ok and Err
- Build match branches with proper scope isolation
- Mark synthetic patterns as used to avoid unused variable warnings
- Create e_lookup_local, e_tag, and e_return expressions for branch bodies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/fx/string_pattern_matching.roc

@@ -0,0 +1,19 @@
+app [main!] { pf: platform "./platform/main.roc" }
+
+import pf.Stdout
+
+# Tests pattern matching on string literals in match expressions.
+
+main! = || {
+    greet("Alice")
+    greet("Bob")
+}
+
+greet = |name| {
+    message = match name {
+        "Alice" => "Hello Alice!"
+        "Bob" => "Hey Bob!"
+        _ => "Hello stranger!"
+    }
+    Stdout.line!(message)
+}