### 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
break;
				case 'f':
					escaped = '\f';
					break;
				case 'n':
					escaped = '\n';
					break;
				case 'r':
					escaped = '\r';
					break;
				case 't':
					escaped = '\t';
					break;
				case 'v':
					escaped = '\v';
					break;
				case '\'':
					escaped = '\'';
					break;
				case '\"':
					escaped = '\"';
					break;
				case '\\':
					escaped = '\\';
					break;
				case 'U':
				case 'u': {
					
					int hex_len = (code == 'U') ? 6 : 4;
					for (int j = 0; j < hex_len; j++) {