### 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 (has_error()) {
		return pop_error();
	}
	_skip_whitespace();
	if (pending_newline) {
		pending_newline = false;
		if (!multiline_mode) {
			
			return last_newline;
		}
	}
	
	if (has_error()) {
		return pop_error();
	}
	_start = _current;
	start_line = line;
	start_column = column;
	leftmost_column = column;
	rightmost_column = column;
	if (pending_indents != 0) {
		
		_start -= start_column - 1;
		start_column = 1;
		leftmost_column = 1;