### 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_unicode_identifier_start(_peek())) {
		_advance(); 
	} else {
		push_error("Expected annotation identifier after \"@\".");
	}
	while (is_unicode_identifier_continue(_peek())) {
		
		_advance();
	}
	Token annotation = make_token(Token::ANNOTATION);
	annotation.literal = StringName(annotation.source);
	return annotation;
}
#define KEYWORDS(KEYWORD_GROUP, KEYWORD)     \
	KEYWORD_GROUP('a')                       \
	KEYWORD("as", Token::AS)                 \
	KEYWORD("and", Token::AND)               \
	KEYWORD("assert", Token::ASSERT)         \
	KEYWORD("await", Token::AWAIT)           \
	KEYWORD_GROUP('b')                       \
	KEYWORD("break", Token::BREAK)           \
	KEYWORD("breakpoint", Token::BREAKPOINT) \
	KEYWORD_GROUP('c')                       \
	KEYWORD("class", Token::CLASS)           \
	KEYWORD("class_name", Token::CLASS_NAME) \
	KEYWORD("const", Token::CONST)           \
	KEYWORD("continue", Token::CONTINUE)     \