Add a Meta API for vhdl_syntax (#451)

This adds a const META: &'static Layout field to the AstNode.
The meta field contains static information about the layout of a node. For example, for a SyntaxNode, it contains all fields with flags whether the field is repeated or optional.
This enables validator APIs and potentially tests and generally computes more fields instead of generating them.

Author: Schottkyc137

vhdl_syntax/src/syntax/generated/concurrent_statements.rs

@@ -31,3 +31,5 @@ pub mod types;
 pub use types::*;
 pub mod builders;
 pub use builders::*;
+pub mod meta;
+pub use meta::*;