Refs #18687. Add custom stg example

Signed-off-by: Ricardo González Moreno <[email protected]>

resources/Custom.stg

@@ -0,0 +1,47 @@
+main(ctx, definitions) ::= <<
+
+$definitions; separator="\n"$
+
+>>
+
+module(ctx, parent, module, definition_list) ::= <<
+
+$definition_list$
+
+>>
+
+definition_list(definitions) ::= <<
+
+$definitions; separator="\n"$
+
+>>
+
+annotation(ctx, annotation) ::= <<>>
+
+const_decl(ctx, parent, const) ::= <<>>
+
+typedef_decl(ctx, parent, typedefs) ::= <<>>
+
+fwd_decl(ctx, parent, type) ::= <<>>
+
+struct_type(ctx, parent, struct) ::= <<>>
+
+union_type(ctx, parent, union, extensions) ::= <<>>
+
+bitset_type(ctx, parent, bitset, extensions) ::= <<>>
+
+enum_type(ctx, parent, enum) ::= <<>>
+
+bitmask_type(ctx, parent, bitmask) ::= <<>>
+
+interface(ctx, parent, interface, export_list) ::= <<>>
+
+export_list(exports) ::= <<>>
+
+exception(ctx, parent, exception) ::= <<>>
+
+operation(ctx, parent, operation, param_list) ::= <<>>
+
+param_list(parameters) ::= <<>>
+
+param(parameter) ::= <<>>