### 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
error.start_column = column;
		error.leftmost_column = column;
		error.end_column = column + 1;
		error.rightmost_column = column + 1;
		push_error(error);
		has_error = true;
	} else if (is_unicode_identifier_start(_peek()) || is_unicode_identifier_continue(_peek())) {
		
		push_error("Invalid numeric notation.");
	}
	
	int len = _current - _start;
	String number = String(_start, len).replace("_", "");
	
	if (base == 16) {
		int64_t value = number.hex_to_int();
		return make_literal(value);
	} else if (base == 2) {
		int64_t value = number.bin_to_int();
		return make_literal(value);
	} else if (has_decimal || has_exponent) {
		double value = number.to_float();
		return make_literal(value);
	} else {
		int64_t value = number.to_int();
		return make_literal(value);
	}
}
GDScriptTokenizer::Token GDScriptTokenizer::string() {
	enum StringType {