Improve Composer + PHPCS

[GaryJones]

See the individual commits for more detailed information.

[aduth]

Trying to composer install from an existing installation resulted in an error for me:

[Symfony\Component\Process\Exception\ProcessFailedException]                                                                                                                                           
  The command "'/Users/andrew/Documents/Code/vvv/www/editor/htdocs/wp-content/plugins/gutenberg/vendor/bin/phpcs' '--config-show' 'installed_paths'" failed.                                             
  Exit Code: 255(Unknown error)                                                                                                                                                                          
  Working directory: /Users/andrew/Documents/Code/vvv/www/editor/htdocs/wp-content/plugins/gutenberg                                                                                                     
  Output:                                                                                                                                                                                                
  ================                                                                                                                                                                                       
  Warning: require(/Users/andrew/Documents/Code/vvv/www/editor/htdocs/wp-content/plugins/gutenberg/vendor/composer/../symfony/polyfill-mbstring/bootstrap.php): failed to open stream: No such file or   
  directory in /Users/andrew/Documents/Code/vvv/www/editor/htdocs/wp-content/plugins/gutenberg/vendor/composer/autoload_real.php on line 66       

But after clearing out my vendor directory and trying again, it worked fine.

Looks good 👍

[ntwb]

@GaryJones Why switch composer.json to spaces?

We explicitly changed it to use tabs in #1067 (comment)

Edit: I didn't read the commit message "Four spaces is what Composer uses itself when updating composer.json, so for consistency, it makes to enforce this when editing it manually as well.". We should be consistent with our coding standards and tabs usage where we can and that is to use tabs for indentation everywhere where possible including composer.json 😄

[aduth]

Having more experience with package.json which had done the same, it doesn't seem a worthwhile use of time to try to avoid the automated install tools and maintain the file's indentation manually.

We already allow the two-space in package.json:

gutenberg/.editorconfig

Lines 11 to 13 in a2756aa

	[{package.json,*.yml}]
	indent_style = space
	indent_size = 2