Evaluate the vimeo/psalm dev dependency

[TravisCarden]

• Part of Evaluate dev dependencies #78.

Dependency	vimeo/psalm (via psalm/phar)
Description	A static analysis tool for finding errors in PHP applications
Value/justification	Medium high. Particularly helpful for finding type safety issues, docblock mismatches, and a small number of other issues that PHPStan doesn't flag. Includes a few security-related checks.
Usage/popularity	Very active. Responsive maintainers. Frequent, well-documented releases adhere strictly to Semver.
Security policy	Apparently none. Requested one at vimeo/psalm#9447.
Maintainance	Active. Responsive maintainers. Very frequent, well-documented releases adhere strictly to Semver.
Considerations	The maintainers of PHPStan recommend its use: PHPStan compared to other static analysis tools phpstan/phpstan#8986. Because it's a shim (i.e., a PHAR), it adds no dependencies on other packages and has essentially no effect on the dependency tree.

Dependency tree

$ composer info --tree psalm/phar
psalm/phar 5.6.0 Composer-based Psalm Phar
└──php ^7.1 || ^8.0

[TravisCarden]

Removed in #292. I can't remember the last time it flagged something that PHPStan didn't also catch.