implement FormatString for Java 15 String#formatted

[codylerum]

Here is an initial pass at an implementation for java 15 and up String.formatted

What I'm doing here is detecting early that this is a formatted call on a string instance and just grabbing the value and all of the arguments and sending them directly to a new validate method.

The old validate method is renamed prepareAndValidate and does the work of extracting the pattern and arguments from the argument this for your traditional String#format calls

fixes #2726

[cushon]

Thanks for the PR!

[codylerum]

@cushon That worked well and is a much better way of getting the pattern. I'm just not familiar enough with all the tools available when parsing the source code.

I squashed everything into one commit and updated the formatting and import order.