mruby-compiler: accept double-quoted strings in case/in patterns

The p_value rule only accepted bare tSTRING tokens, which the lexer
emits for single-quoted strings.  Double-quoted strings emit
tSTRING_BEG ... tSTRING (or with interpolation, tSTRING_BEG
string_rep tSTRING), so

  case "hello"
  in "hello"
    :match
  end

raised "syntax error, unexpected string literal" at the `"` after
`in`.  Use the existing `string` non-terminal instead of bare
tSTRING, which also enables alternation (`"a" | "b"`), interpolation
(`"hel#{x}"`), and concatenation by juxtaposition in patterns.

close #6830

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/parse.y

@@ -3987,9 +3987,9 @@ p_value         : p_var
                     {
                       $$ = new_pat_value(p, $1);
                     }
-                | tSTRING
+                | string
                     {
-                      $$ = new_pat_value(p, new_str(p, list1($1)));
+                      $$ = new_pat_value(p, new_str(p, $1));
                     }
                 | keyword_nil
                     {

test/t/syntax.rb

@@ -981,6 +981,32 @@ assert('pattern matching - basic case/in') do
   assert_equal :other, result
 end
 
+assert('pattern matching - string literal patterns') do
+  result = case "hello"
+           in "hello" then :match
+           end
+  assert_equal :match, result
+
+  # double-quoted vs single-quoted should both work
+  result = case 'world'
+           in "world" then :double
+           end
+  assert_equal :double, result
+
+  # alternation with strings
+  result = case "ab"
+           in "ab" | "cd" then :alt
+           end
+  assert_equal :alt, result
+
+  # string interpolation in pattern
+  expected = "lo"
+  result = case "hello"
+           in "hel#{expected}" then :interp
+           end
+  assert_equal :interp, result
+end
+
 assert('pattern matching - array patterns') do
   # simple array pattern
   case [1, 2, 3]