Add support for running Psalm

[joehoyle]

Add support for Psalm + Psalm Plugin WordPress. This is a bit different to other "linters" as this is more of a typechecker, but I think it fits the general paradigm well, and in project that use Psalm, we can at least run it in those contexts. In the future we might want to expand this to run a default config for things like taint analysis.

The major difference with Psalm is we need to install Composer dependencies as those are part of the type flow, and needed for Psalm scanning. At the moment I just installed production dependencies, however I'm not sure on the best route here. It wouldn't be uncommon to include your tests and the like in the Psalm config, in which case you're likely to also need the composer dev deps. I think it would be ok to not run Psalm against tests, but we'd need to somehow exclude them from any Psalm configuration.

Along with this, there's a couple other things needed that hm-linter doesn't directly include (maybe we should also address this at some point)

• composer.phar needs to be pushed to s3://hm-linter/bin/composer
• Base Psalm + Psalm-plugin-wordpress copied to s3://hm-linter/standards/psalm-x.zip
• Some other rigging up to make sure the above are correctly downloaded, and Psalm is only run if you have a psalm.xml in the project

[ntwb]

Another consideration is to allow custom stubs, projects may have a stubs file for the project code.

This is also similar to being able to also add custom PHPCS rules sets not included in HM CS

So, having the ability for Composer to be able to install these custom deps per project would be great

Edit: Just realised this is a pull request and not an issue, the workaround for the above which would still be possible I think in the context of this PR would be a custom S3 deployment of HM CS, this was discussed in Slack last week, so don't worry about my comments here preventing this PR from being merged 😏