### 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
"VCS conflict marker", 
	"`", 
	"?", 
	
	"Error", 
	"End of file", 
};
static_assert(sizeof(token_names) / sizeof(token_names[0]) == GDScriptTokenizer::Token::TK_MAX, "Amount of token names don't match the amount of token types.");
const char *GDScriptTokenizer::Token::get_name() const {
	ERR_FAIL_INDEX_V_MSG(type, TK_MAX, "<error>", "Using token type out of the enum.");
	return token_names[type];
}
bool GDScriptTokenizer::Token::is_identifier() const {
	
	
	switch (type) {
		case IDENTIFIER:
		case MATCH: 
		
		case CONST_PI:
		case CONST_INF:
		case CONST_NAN:
		case CONST_TAU:
			return true;
		default:
			return false;
	}
}
bool GDScriptTokenizer::Token::is_node_name() const {