Add a few more special literals

The grammar hardcodes a few directive-looking things that we don't
handle today, namely playground literals and key path strings. Adding
those in explicitly for now, but this is worth reevaluating in the
future; it may be better to go generic and let the real language be more
restrictive.

Fixes #43

corpus/expressions.txt

@@ -496,6 +496,8 @@ compoundValue[keyPath: \.self] = (a: 10, b: 20)
 
 let nestedKeyPath = \OuterStructure.outer.someValue
 
+let keyPathStringExpression = #keyPath(someProperty)
+
 ---
 
 (source_file
@@ -567,7 +569,13 @@ let nestedKeyPath = \OuterStructure.outer.someValue
  (navigation_suffix
  (simple_identifier)))
  (navigation_suffix
- (simple_identifier)))))
+ (simple_identifier))))
+ (property_declaration
+ (value_binding_pattern
+ (non_binding_pattern
+ (simple_identifier)))
+ (key_path_string_expression
+ (simple_identifier))))
 
 ===
 Tuple expression in function

corpus/literals.txt

@@ -322,3 +322,24 @@ let infinity = "\u{221E}"
  (non_binding_pattern
  (simple_identifier)))
  (line_string_literal)))
+
+===
+Playground literals
+===
+
+===
+/tmp/top-repos/firefox/Client/Frontend/Update/UpdateViewModel.swift
+===
+
+let playgroundLiteral = #imageLiteral(resourceName: "heart")
+
+---
+
+(source_file
+ (property_declaration
+ (value_binding_pattern
+ (non_binding_pattern
+ (simple_identifier)))
+ (simple_identifier)
+ (line_string_literal)))
+