### 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
KEYWORD("trait", Token::TRAIT)           \
	KEYWORD_GROUP('v')                       \
	KEYWORD("var", Token::VAR)               \
	KEYWORD("void", Token::VOID)             \
	KEYWORD_GROUP('w')                       \
	KEYWORD("while", Token::WHILE)           \
	KEYWORD_GROUP('y')                       \
	KEYWORD("yield", Token::YIELD)           \
	KEYWORD_GROUP('I')                       \
	KEYWORD("INF", Token::CONST_INF)         \
	KEYWORD_GROUP('N')                       \
	KEYWORD("NAN", Token::CONST_NAN)         \
	KEYWORD_GROUP('P')                       \
	KEYWORD("PI", Token::CONST_PI)           \
	KEYWORD_GROUP('T')                       \
	KEYWORD("TAU", Token::CONST_TAU)
#define MIN_KEYWORD_LENGTH 2
#define MAX_KEYWORD_LENGTH 10
#ifdef DEBUG_ENABLED
void GDScriptTokenizer::make_keyword_list() {
#define KEYWORD_LINE(keyword, token_type) keyword,
#define KEYWORD_GROUP_IGNORE(group)
	keyword_list = {
		KEYWORDS(KEYWORD_GROUP_IGNORE, KEYWORD_LINE)
	};
#undef KEYWORD_LINE
#undef KEYWORD_GROUP_IGNORE
}
#endif 
GDScriptTokenizer::Token GDScriptTokenizer::potential_identifier() {