### 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 (_peek() != '\n') {
						
						
						result += ch;
						_advance();
						break;
					}
					[[fallthrough]];
				case '\n':
					
					newline(false);
					valid_escape = false; 
					break;
				default:
					Token error = make_error("Invalid escape in string.");
					error.start_column = column - 2;
					error.leftmost_column = error.start_column;
					push_error(error);
					valid_escape = false;
					break;
			}
			
			if (valid_escape) {
				if ((escaped & 0xfffffc00) == 0xd800) {
					if (prev == 0) {
						prev = escaped;
						prev_pos = column - 2;
						continue;
					} else {
						Token error = make_error("Invalid UTF-16 sequence in string, unpaired lead surrogate");