Enforcing Code Style

[James Daugherty <jd...@jdresources.net.INVALID>]

Hi Everyone,

A while ago we discussed adopting Spotless & Checkstyle (
https://github.com/apache/grails-core/issues/13754 & several weekly
meetings) for linting & validation.  I've gone ahead and applied the Grails
forge code style to the grails-core code base in this PR[1].

Please note, the checkstyle code validations are only for the java files.
Separately, I intend to apply codenarc for groovy validation.  This PR does
format all non-test code however.  I encourage people to take a look at
this branch since it will be enforced at build time after merge.

As a refresher, these were the notes on this checkstyle configuration:

Checkstyle configuration that checks the sun coding conventions from:
- the Java Language Specification at
  http://java.sun.com/docs/books/jls/second_edition/html/index.html
- the Sun Code Conventions at http://java.sun.com/docs/codeconv/
- the Javadoc guidelines at
  http://java.sun.com/j2se/javadoc/writingdoccomments/index.html
- the JDK Api documentation http://java.sun.com/j2se/docs/api/index.html
- some best practices


If people want to change this configuration,we can easily rebase & mass
reformat the associated commit.  Moreover, we can ignore this commit (and
others) via the git config `blame.ignoreRevsFile`.  I intend to check in
this file under /etc/git so people can update this file to have an easy way
to use git blame without seeing reformatting changes.

Please take a look given the impact this has on our project.

[1] https://github.com/apache/grails-core/pull/14903