- introduce `Zend_Loader::tryLoadClass()` to attempt loading a class but do not throw an exception when file is not found (but still throw when required class in the existing file is not defined)
- did not go for suppressing loading classes with `@` suppressor by default, since regular warnings/errors coming from a loaded file should be visible, otherwise it might be very confusing for devs
- introduce `Zend_Loader_Exception_FileNotFoundException` and `Zend_Loader_Exception_ClassNotFoundException`
  - instead of throwing `Zend_Exception` in `Zend_Loader::loadClass()` with a generic message `File \"$file\" does not exist or class \"$class\" was not found in the file`
  - `Zend_Loader_Exception_FileNotFoundException` will be thrown with message `File "$file" could not be found within configured include_path.` or
  - `Zend_Loader_Exception_ClassNotFoundException` with message `Class "$class" was not found in the file "$file".`, in their respective cases.
- since composer autoloader takes precedence and `Zend_Autoloader` / `Zend_Loader` is only an extra helper, when necessary (when still needed and configured on legacy projects) - `Zend_Loader` should not emit warnings when checking for files, by default. Added `isReadable` check inside `loadFile` before `include`/`include_once`. There might be a performance hit, but it should be okay when most of the autoloading is handled by composer autoloader anyway.
- Zend_Loader::loadClass returns boolean now
- adjust combinations of `class_exists` + `Zend_Loader::loadClass` in remaining affected places (i.a. Translate or File_Transfer or Filter adapters)
- unregister autoloader from spl_autoload when `Zend_Loader_Autoloader::resetInstance()` is called
- added more tests for Zend_Loader and Zend_Autoloader
- added tests for loading custom Zend_Translate adapter while having Zend_Autoloader enabled
- adjusted existing tests which expected "file not found" php warning to be triggered by `Zend_Loader`

ref: 76477fb

fixes the test failure on php 7.4 & 8.0:
- Failed asserting that PHPUnit_Framework_Error_Notice Object (...) is an instance of class "Zend_Gdata_App_Exception".

in isReadable() - it should be more performant and the function is available since php 5.3.2