Prepend underscore to numerical enum variants

[vlisivka]

After stripping of enum prefix from enum variant name, some constants may have invalid name because they start with number. e.g. Time3s = 0, -> 3s = 0, . As workaround, prepend _ to name of such constants, e.g. Time3s = 0, -> _3s = 0, .

[Arkanayan]

It is also an issue where the variant name starts with underscore followed by a number. e.g.

enum MyEnumSize {
	E_UNKNOWN = 0;
	_123 = 1;
	_456 = 2;
	_789 = 3;
	_7890 = 4;
}

to

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, ::prost::Enumeration)]
#[repr(i32)]
pub enum MyEnumSize {
    EUnknown = 0,
    123 = 1,
    456 = 2,
    789 = 3,
    7890 = 4,
}

^ which is invalid.