bshell: re-organise build into three separate components

bshell: the front-end binary
bshell.runtime: contains the parser, compiler, and classes needed to run bshell scripts
bshell.core: contains the builtin commandlets and aliases

bshell.runtime/ast/null.c

@@ -0,0 +1,7 @@
+#include <bshell/ast.h>
+#include <bshell/parse/token.h>
+
+struct bshell_ast_node_definition null_bshell_ast_node = {
+	.def_id = BSHELL_AST_NULL,
+	.def_node_size = sizeof(struct bshell_null_ast_node),
+};