[3.0] Extend template system with callable support

[dragomano]

Adds support for callable strings (e.g., Class::method#) in the loadSubTemplate method, allowing both static methods and instance method calls with object caching. Adds the same support for the callback option in Admin.template.php

Example with callback functions:

<?php

namespace SMF;

class SomeClass
{
	public static function getConfigVars(): array
	{
		return [
			['callback', 'legacy_callback_name'], // looking for `template_callback_legacy_callback_name` function
			['callback', __CLASS__ . '::test#'], // looking for `test` method in the current class
			['callback', TestClass::class . '::test#'], // looking for `test` method in other class
			['callback', TestClass::class . '::test2'], // looking for `test2` static method in other class
		];
	}

	public function test(): void
	{
		echo __METHOD__;
	}
}

class TestClass
{
	public function test(): void
	{
		echo __METHOD__;
	}

	public static function test2(): void
	{
		echo __METHOD__;
	}
}

Example with subaction:

Utils::$context['sub_template'] = 'show_settings'; // current (legacy)
Utils::$context['sub_template'] = TestClass::class . '::test#'; // new

Example with layers:

Utils::$context['template_layers'][] = TestClass::class . '::test#';

<?php

namespace SMF;

class TestClass
{
	public function testAbove(): void
	{
		echo 'above';
	}

	public function testBelow(): void
	{
		echo 'below';
	}
}

[live627]

Should similar code in the profile area also be updated?

[Sesquipedalian]

This is a cool idea, but we might end up making profound changes to the theme system in 3.0. For that reason, I recommend that you don't invest much effort into theme-related code until that has been settled.