Fields beginning with _{digit} generate invalid C# code

[jskeet]

A proto like this:

Generates C# code like this:

public const int 0FieldNumber = 1;
private bool has0;
private string 0_ = "";
public bool Has0 {
  get { return has0; }
}
public string 0 {
  get { return 0_; }
}

The 0FieldNumber, 0 and 0_ field/property names are invalid.

It looks like the Java code generator creates invalid code for this too, so although this is a regression in the C# codegen compared with the protobuf-csharp-port generator, we might deem it a reasonable restriction - but it would probably make sense to prohibit it explicitly.

[jskeet]

In the protobuf-csharp-port codebase, this was fixed by
jskeet/protobuf-csharp-port@1851676