Fix PHPStan handling of plugin constants defined via function calls

[daledupreez]

Changes proposed in this Pull Request:

This PR works around a bug or limitation in PHPStan where it does not recognise constants defined using the return value from a function. The issue was resulting in the WC_STRIPE_PLUGIN_URL and WC_STRIPE_PLUGIN_PATH constants not being found/seen by PHPStan. To work around the issue, I've added conditional definitions for those two constants in phpstan-stubs/wc_stripe_constants.php so they get picked up during PHPStan analysis runs. Note that the values are not correct, as we need to use constant values for both.

I have also updated the PHPStan baseline to remove the previously tracked errors stemming from the "missing" constants.

Testing instructions

Code review should be good enough on this one, and the PHPStan analysis should be clean.

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• [N/A] Covered with tests (or have a good reason not to test in description ☝️)
• [N/A] Tested on mobile (or does not apply)

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Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[malithsen]

LGTM

[malithsen]

Is this check needed for PHPStan? It might give the impression that this is a WP runtime file.