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, 8 insertions, 0 deletions

diff --git a/vm.c b/vm.c
index 079809e05f..f82523ad4e 100644
--- a/vm.c
+++ b/vm.c
@@ -4256,6 +4256,14 @@ rb_ruby_verbose_ptr(void)
     return &cr->verbose;
 }
 
+static bool prism;
+
+bool *
+rb_ruby_prism_ptr(void)
+{
+    return &prism;
+}
+
 VALUE *
 rb_ruby_debug_ptr(void)
 {