While there is value in declaring `DateTimeImmutable::createFromInterface()` as mutation-free in
a stub, this method was introduced in PHP 8.0, so we cannot really declare it in a stub.
For now, we drop it, as the value of its stub declaration is much lower than the problems it
introduces through its conditional existence.
Also:
* a non-empty format string will always lead to `non-empty-string`
* it seems that you can throw **everything** at `DateTimeInterface#format()`, even null bytes,
and it will always produce a `string`
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3353-L3369)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3596-L3612)
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3308-L3324)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3549-L3565)
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3258-L3274)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3496-L3512)
Also simplified the return type from `static|false` to `static`, since
the method throws at all times, on failure.
On PHP 7.x, it could only fail if an invalid type was passed in, which is
not really valid anyway, from a type perspective.
Ref (PHP 8.1.x): 32d55f7422/ext/date/php_date.c (L3212-L3228)
Ref (PHP 7.0.33): bf574c2b67/ext/date/php_date.c (L3447-L3463)
Also expanded the return type from `static` to `static|false`, since the
operation can fail (with a warning too), such as in following example:
https://3v4l.org/Xrjlc
```php
<?php
var_dump(
(new DateTimeImmutable())
->modify('potato')
);
```
Produces
```
Warning: DateTimeImmutable::modify(): Failed to parse time string (potato) at position 0 (p): The timezone could not be found in the database in /in/Xrjlc on line 6
bool(false)
```
Ref: 534127d3b2/ext/date/php_date.stub.php (L508-L509)
Note: also verified that a `DateTimeImmutable#getTimezone()` always returns
a default timezone (initialized internally), and therefore restricted the
type a bit.
