Improve documentation and types in PLL_Model

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Fixes https://github.com/polylang/polylang-pro/issues/1314.

This improves documentation in PLL_Model and fixes issues reported by PHPStan about types.

Most of the work has been done around PLL_Model->get_languages_list(), by creating a new PHPStan extension.

About @phpstan-* tags:
I added several @phpstan-param and @phpstan-return tags to specify types more precisely. This could have been done directly in the classic @param and @return tags but I prefered to keep these ones simpler and easier to read.
Example, post IDs are positive integers, the integer's range is specified:

@return int[] A list of post IDs.

@phpstan-return array<int<1,max>>

Several other fixes have been made in other files, following the changes in PLL_Model.

PLL_Model's tests are now using assertSame() instead of assertEquals().

Note: rollback composer.json once the PHPStan extension is merged into master.