phpstan/extension-installer should be a composer requirement

[emteknetnz]

"phpstan/extension-installer": "^1.3" was added to module-standardiser as a dev-dependency, along with silverstripe/standards - https://github.com/silverstripe/module-standardiser/pull/33/files#diff-6ea5c1ded94eb095c179fa17eb437bc66833529ec9bc1e34d1bd16853c2ddbd9R44

We should simply make phpstan/extension-installer a composer requirement of silverstripe/standards and remove phpstan/extension-installer as dev dependency from modules. The use case for keeping this optional is very narrow and as far as I know it doesn't matter if it's installed regardless. So we should just simplify things and put it in there.

If we do this, also update module-standariser and run it