fix: allow ident name start with _

[Ggiggle]

Motivation

The rust name can start with "_", so we should remove this strict for idl parser.

enum _Enum {
    _1 = 1,
    _2 = 2
}

codegen should be

impl _Enum {
            pub const _1: Self = Self(1);
            pub const _2: Self = Self(2);

            pub fn inner(&self) -> i32 {
                self.0
            }
    ...
}

Solution

Keep the starting '_'* for idl item name.