### Godot version

4.0

### System information

All OS/Hardware

### Issue description

Not sure if it's a bug / inconsistency or a willingness 😄

When writing numbers in GDScript, we can use `_` character to make number more readable for a human. Rules for this character differ depending on where it is used : before or in decimal part, making its usage inconsistent. 
- When use before decimal part, only an underscore can not follow another undersocre
- In decimal part, multiple underscores can be write without any complaint of tokenizer



### Steps to reproduce

Here is test functions to illustrate

```gdscript
func test_number_allowed_without_error() -> void:
    var a:int = 1_2_3_4_5_6
    var b:int = 123_456
    var c:int = 1.2_3_4_5_6
    var d:int = 1.2_____3_____4____5_____6
    var e:int = 1._2_3_4_5
    var f:int = 1.________23456

func test_number_not_allowed() -> void:
    var a:int = 1__2
    var b:int = 1__2.1__2
    var c:int = _1.23456
```

With a screen to show editor syntax errors

![image](https://user-images.githubusercontent.com/121614863/220851058-38439a3f-3ba8-40fa-9556-a0faf045c943.png)

### Minimal reproduction project

N/A
if (!is_digit(_peek())) {
				Token error = make_error(R"(Expected exponent value after "e".)");
				error.start_column = column;
				error.leftmost_column = column;
				error.end_column = column + 1;
				error.rightmost_column = column + 1;
				push_error(error);
			}
			previous_was_underscore = false;
			while (is_digit(_peek()) || is_underscore(_peek())) {
				if (is_underscore(_peek())) {
					if (previous_was_underscore) {
						Token error = make_error(R"(Only one underscore can be used as a numeric separator.)");
						error.start_column = column;
						error.leftmost_column = column;
						error.end_column = column + 1;
						error.rightmost_column = column + 1;
						push_error(error);
					}
					previous_was_underscore = true;
				} else {
					previous_was_underscore = false;
				}
				_advance();
			}
		}
	}
	
	if (!has_error && has_decimal && _peek() == '.' && _peek(1) != '.') {
		Token error = make_error("Cannot use a decimal point twice in a number.");