fix(switcher): migrate extension for Polylang 3.9

[Hug0-Drelon]

What?

• Remove PLL_Switcher::the_languages() return-type extension (class removed in Polylang 3.9).
• Update pll_the_languages() inference for raw, echo, and default void return.
• Add DeprecatedPllSwitcherRule and a scan-only stub to report legacy PLL_Switcher usage.
• Refresh PHPUnit coverage and add a local tests/phpstan.neon for extension development.

Why?

Polylang 3.9 refactored the language switcher: PLL_Switcher is gone, pll_the_languages() now returns void|string|array depending on $args, and the new WP_Syntex\Polylang\Switcher\Switcher class is typed via stubs/source.

The previous extension targeted a removed API and produced *ERROR* types once updated stubs dropped PLL_Switcher.

How?

• Replace GuessTypeFromSwitcherAttributes with GuessTypeFromSwitcherArgs, using PHPStan 2 APIs (getConstantArrays(), getConstantStrings()).
• Register DeprecatedPllSwitcherRule (polylang.deprecatedSwitcher, polylang.deprecatedSwitcherMethod).
• Load stubs/pll-switcher-deprecated.php via scanFiles so the deprecated symbol remains discoverable.
• Declare phpstan/phpstan as a direct runtime dependency.

Test plan

• composer test (34 PHPUnit tests)
• composer stan (self-analysis of switcher inference code)
• Consumer check: run PHPStan in polylang/ after bumping this package