- Fix an error when using heredoc with non-word delimiters

I've found #986 regressions in the following RuboCop CI matrix:
https://github.com/rubocop/rubocop/actions/runs/7437077900/job/20234293932?pr=12598

So, #986 was not a sufficient fix. This PR fixes an error when using heredoc with non-word delimiters:

```ruby
<<~'+'
  foo
+
```

The following repro test have been newly added:

```console
$ bundle exec ruby -Itest test/test_lexer.rb
(snip)

  1) Error:
TestLexer#test_heredoc_plus_character:
RegexpError: target of repeat operator is not specified: /+\z/
    /Users/koic/src/github.com/whitequark/parser/lib/parser/lexer/literal.rb:250:in `delimiter?'
    /Users/koic/src/github.com/whitequark/parser/lib/parser/lexer/literal.rb:138:in `nest_and_try_closing'
    /Users/koic/src/github.com/whitequark/parser/lib/parser/lexer-strings.rb:4165:in `advance'
    /Users/koic/src/github.com/whitequark/parser/lib/parser/lexer-F1.rb:11369:in `advance'
    test/test_lexer.rb:81:in `assert_scanned'
    test/test_lexer.rb:1001:in `test_heredoc_plus_character'
```

lib/parser/lexer/literal.rb

@@ -247,7 +247,7 @@ def delimiter?(delimiter)
         # E
         # because there are not enough leading spaces in the closing delimiter.
         delimiter.end_with?(@end_delim) &&
-          delimiter.sub(/#{@end_delim}\z/, '').bytes.all? { |c| c == SPACE }
+          delimiter.sub(/#{Regexp.escape(@end_delim)}\z/, '').bytes.all? { |c| c == SPACE }
       elsif @indent
         @end_delim == delimiter.lstrip
       else

test/test_lexer.rb

@@ -997,6 +997,16 @@ def test_heredoc_one_character
                    :tNL,             nil,        [7, 8])
   end
 
+  def test_heredoc_plus_character
+    assert_scanned("a = <<'+'\nABCDEF\n+\n",
+                   :tIDENTIFIER,     "a",        [0, 1],
+                   :tEQL,            "=",        [2, 3],
+                   :tSTRING_BEG,     "<<\'",     [4, 9],
+                   :tSTRING_CONTENT, "ABCDEF\n", [10, 17],
+                   :tSTRING_END,     "+",        [17, 18],
+                   :tNL,             nil,        [9, 10])
+  end
+
   def test_heredoc_cr
     assert_scanned("a = <<E\r\r\nABCDEF\r\r\nE\r\r\r\n",
                    :tIDENTIFIER,     "a",          [0, 1],