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, 31 insertions, 15 deletions

diff --git a/load.c b/load.c
index 35b80a6b4a..71bd3c7c1e 100644
--- a/load.c
+++ b/load.c
@@ -737,21 +737,37 @@ load_iseq_eval(rb_execution_context_t *ec, VALUE fname)
     const rb_iseq_t *iseq = rb_iseq_load_iseq(fname);
 
     if (!iseq) {
-        rb_execution_context_t *ec = GET_EC();
-        VALUE v = rb_vm_push_frame_fname(ec, fname);
-        rb_ast_t *ast;
-        VALUE parser = rb_parser_new();
-        rb_parser_set_context(parser, NULL, FALSE);
-        ast = (rb_ast_t *)rb_parser_load_file(parser, fname);
-
-        rb_thread_t *th = rb_ec_thread_ptr(ec);
-        VALUE realpath_map = get_loaded_features_realpath_map(th->vm);
-
-        iseq = rb_iseq_new_top(&ast->body, rb_fstring_lit("<top (required)>"),
-                               fname, realpath_internal_cached(realpath_map, fname), NULL);
-        rb_ast_dispose(ast);
-        rb_vm_pop_frame(ec);
-        RB_GC_GUARD(v);
+        if (*rb_ruby_prism_ptr()) {
+            pm_string_t input;
+            pm_options_t options = { 0 };
+
+            pm_string_mapped_init(&input, RSTRING_PTR(fname));
+            pm_options_filepath_set(&options, RSTRING_PTR(fname));
+
+            pm_parser_t parser;
+            pm_parser_init(&parser, pm_string_source(&input), pm_string_length(&input), &options);
+
+            iseq = rb_iseq_new_main_prism(&input, &options, fname);
+
+            pm_string_free(&input);
+            pm_options_free(&options);
+        } else {
+            rb_execution_context_t *ec = GET_EC();
+            VALUE v = rb_vm_push_frame_fname(ec, fname);
+            rb_ast_t *ast;
+            VALUE parser = rb_parser_new();
+            rb_parser_set_context(parser, NULL, FALSE);
+            ast = (rb_ast_t *)rb_parser_load_file(parser, fname);
+
+            rb_thread_t *th = rb_ec_thread_ptr(ec);
+            VALUE realpath_map = get_loaded_features_realpath_map(th->vm);
+
+            iseq = rb_iseq_new_top(&ast->body, rb_fstring_lit("<top (required)>"),
+                                   fname, realpath_internal_cached(realpath_map, fname), NULL);
+            rb_ast_dispose(ast);
+            rb_vm_pop_frame(ec);
+            RB_GC_GUARD(v);
+        }
     }
     rb_exec_event_hook_script_compiled(ec, iseq, Qnil);
     rb_iseq_eval(iseq);