### 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
bool only_ascii = _peek(-1) < 128;
	
	while (is_unicode_identifier_continue(_peek())) {
		char32_t c = _advance();
		only_ascii = only_ascii && c < 128;
	}
	int len = _current - _start;
	if (len == 1 && _peek(-1) == '_') {
		
		return make_token(Token::UNDERSCORE);
	}
	String name(_start, len);
	if (len < MIN_KEYWORD_LENGTH || len > MAX_KEYWORD_LENGTH) {
		
		return make_identifier(name);
	}
	
#define KEYWORD_GROUP_CASE(char) \
	break;                       \
	case char:
#define KEYWORD(keyword, token_type)                                                                                      \
	{                                                                                                                     \
		const int keyword_length = sizeof(keyword) - 1;                                                                   \
		static_assert(keyword_length <= MAX_KEYWORD_LENGTH, "There's a keyword longer than the defined maximum length");  \