Make fallback extension configurable

[rustamwin]

Q	A
Is bugfix?	✔️
New feature?	✔️
Breaks BC?	❌
Fixed issues	#227

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d211ca6) 99.74% compared to head (c6a0271) 99.74%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #232 +/- ## ========================================= Coverage 99.74% 99.74% - Complexity 299 300 +1 ========================================= Files 18 18 Lines 791 792 +1 ========================================= + Hits 789 790 +1 Misses 2 2 ``` | [Impacted Files](https://app.codecov.io/gh/yiisoft/view/pull/232?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/ViewTrait.php](https://app.codecov.io/gh/yiisoft/view/pull/232?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL1ZpZXdUcmFpdC5waHA=) | `100.00% <100.00%> (ø)` | |

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[what-the-diff[bot]]

PR Summary

• Introduction of constant variable for PHP extension For more consistent code, a constant PHP_EXTENSION has been introduced. This adjustment makes our code more resilient against minor changes and less prone to errors.

• Use of new constant in place initialization The property $defaultExtension within ViewTrait has been modified to use the new constant PHP_EXTENSION, replacing the previous static string, 'php'. This change is consistent with the introduction of the new constant. In addition, a new attribute $fallbackExtension has been introduced with a default value of PHP_EXTENSION.

• Addition of backup solution A new method withFallbackExtension was added to the ViewTrait trait. If the selected extension is not available, this method provides an automatic alternative, improving reliability.

• Modifying find methods to adapt to fallback strategy The findTemplateFile method in ViewTrait has been altered to use the $fallbackExtension value rather than the static '.php'. This harmonizes with the above highlights, providing a more flexible and adjustable solution.

• Tests covering new fallback mechanism For verification of newly added functionalities, a new test case testRenderWithoutFileExtension has been added in the ViewTest class. The test uses the withDefaultExtension and withFallbackExtension methods, ensuring that the changes work as intended and improving overall robustness of the software.

[samdark]

👍