### 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 (_is_at_end()) {
							return make_error("Unterminated string.");
						}
						char32_t digit = _peek();
						char32_t value = 0;
						if (is_digit(digit)) {
							value = digit - '0';
						} else if (digit >= 'a' && digit <= 'f') {
							value = digit - 'a';
							value += 10;
						} else if (digit >= 'A' && digit <= 'F') {
							value = digit - 'A';
							value += 10;
						} else {
							
							Token error = make_error("Invalid hexadecimal digit in unicode escape sequence.");
							error.start_column = column;
							error.leftmost_column = error.start_column;
							error.end_column = column + 1;
							error.rightmost_column = error.end_column;
							push_error(error);
							valid_escape = false;
							break;
						}
						escaped <<= 4;
						escaped |= value;
						_advance();
					}
				} break;
				case '\r':