NEW Add script for setting phpstan config

[GuySartorelli]

This PR assumes #32 is merged first, as that changes how is_module() works.

This PR does the following:

• Add new is_theme() function to determine if a module is specifically a theme
• Add new check_dir_exists() function to check if a directory exists or not
• Adds new phpstan.php script for CMS 5 projects
  • is only processed for modules which aren't themes
  • is only processed if one of the relevant directories exists
  • adds phpstan.neon.dist configuration file
  • adds silverstripe/standards and phpstan/extension-installer dev dependencies

Issue

• Validate @method docblock tag in CI .github#171

[emteknetnz]

Suggested change

	* check_file_exists('src')
	* check_dir_exists('src')

[GuySartorelli]

Oops. Fixed.