### 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 - 2;
						error.leftmost_column = error.start_column;
						push_error(error);
						valid_escape = false;
						prev = 0;
					}
				} else if ((escaped & 0xfffffc00) == 0xdc00) {
					if (prev == 0) {
						Token error = make_error("Invalid UTF-16 sequence in string, unpaired trail surrogate");
						error.start_column = column - 2;
						error.leftmost_column = error.start_column;
						push_error(error);
						valid_escape = false;
					} else {
						escaped = (prev << 10UL) + escaped - ((0xd800 << 10UL) + 0xdc00 - 0x10000);
						prev = 0;
					}
				}
				if (prev != 0) {
					Token error = make_error("Invalid UTF-16 sequence in string, unpaired lead surrogate");
					error.start_column = prev_pos;
					error.leftmost_column = error.start_column;
					push_error(error);
					prev = 0;
				}
			}
			if (valid_escape) {
				result += escaped;
			}
		} else if (ch == quote_char) {