### 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
ERR_FAIL_INDEX_V_MSG(p_token_type, Token::TK_MAX, "<error>", "Using token type out of the enum.");
	return token_names[p_token_type];
}
void GDScriptTokenizer::set_source_code(const String &p_source_code) {
	source = p_source_code;
	if (source.is_empty()) {
		_source = U"";
	} else {
		_source = source.ptr();
	}
	_current = _source;
	line = 1;
	column = 1;
	length = p_source_code.length();
	position = 0;
}
void GDScriptTokenizer::set_cursor_position(int p_line, int p_column) {
	cursor_line = p_line;
	cursor_column = p_column;
}
void GDScriptTokenizer::set_multiline_mode(bool p_state) {