feat: add triple-quoted multiline string literals (issue 12)

Support """...""" syntax for multiline strings with:
- Automatic indent stripping (based on minimum indentation)
- Leading newline after opening """ is skipped
- Trailing whitespace-only line before closing """ is stripped
- String interpolation ({expr}) support
- All escape sequences supported
- Formatter outputs multiline strings for strings containing newlines

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/formatter.rs

@@ -733,7 +733,30 @@ impl Formatter {
         match &lit.kind {
             LiteralKind::Int(n) => n.to_string(),
             LiteralKind::Float(f) => format!("{}", f),
-            LiteralKind::String(s) => format!("\"{}\"", s.replace('\\', "\\\\").replace('"', "\\\"").replace('{', "\\{").replace('}', "\\}")),
+            LiteralKind::String(s) => {
+                if s.contains('\n') {
+                    // Use triple-quoted multiline string
+                    let tab = " ".repeat(self.config.indent_size);
+                    let base_indent = tab.repeat(self.indent_level);
+                    let content_indent = tab.repeat(self.indent_level + 1);
+                    let lines: Vec<&str> = s.split('\n').collect();
+                    let mut result = String::from("\"\"\"\n");
+                    for line in &lines {
+                        if line.is_empty() {
+                            result.push('\n');
+                        } else {
+                            result.push_str(&content_indent);
+                            result.push_str(&line.replace('{', "\\{").replace('}', "\\}"));
+                            result.push('\n');
+                        }
+                    }
+                    result.push_str(&base_indent);
+                    result.push_str("\"\"\"");
+                    result
+                } else {
+                    format!("\"{}\"", s.replace('\\', "\\\\").replace('"', "\\\"").replace('{', "\\{").replace('}', "\\}"))
+                }
+            },
             LiteralKind::Char(c) => format!("'{}'", c),
             LiteralKind::Bool(b) => b.to_string(),
             LiteralKind::Unit => "()".to_string(),