Introduce --parser runtime flag

Introduce runtime flag for specifying the parser,

```
ruby --parser=prism
```

also update the description:

```
$ ruby --parser=prism --version
ruby 3.3.0dev (2023-12-08T04:47:14Z add-parser-runtime.. 0616384c9f) +PRISM [x86_64-darwin23]
```

[Bug #20044]

1 files changed, 13 insertions, 0 deletions

diff --git a/test/ruby/test_rubyoptions.rb b/test/ruby/test_rubyoptions.rb
index eb10ed6aec..be363008de 100644
--- a/test/ruby/test_rubyoptions.rb
+++ b/test/ruby/test_rubyoptions.rb
@@ -287,6 +287,19 @@ class TestRubyOptions < Test::Unit::TestCase
     end
   end
 
+  def test_parser_flag
+    warning = /The Prism compiler is currently experimental and compatibility with parse.y is not yet complete. Please report an issues you find on the prism issue tracker./
+
+    assert_in_out_err(%w(--parser=prism -e) + ["puts :hi"], "", %w(hi), warning)
+
+    assert_in_out_err(%w(--parser=parse.y -e) + ["puts :hi"], "", %w(hi), [])
+    assert_norun_with_rflag('--parser=parse.y', '--version', "")
+
+    assert_in_out_err(%w(--parser=notreal -e) + ["puts :hi"], "", [], /unknown parser notreal/)
+
+    assert_in_out_err(%w(--parser=prism --version), "", /\+PRISM/, warning)
+  end
+
   def test_eval
     assert_in_out_err(%w(-e), "", [], /no code specified for -e \(RuntimeError\)/)
   end