NEW restored Extension::__construct()

[fspringveldt]

Fixes #6965

[tractorcow]

you'll need to add parent::__construct() to all the places it was removed, as well.

Also, we've removed the $class property; that shouldn't be re-introduced.

[tractorcow]

One linting bug @fspringveldt

FILE: ...build/silverstripe/silverstripe-framework/src/Core/Extension.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 50 | ERROR | [x] Function closing brace must go on the next line
    |       |     following the body; found 1 blank lines before
    |       |     brace
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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