Enforce `var` vs `FullTypeName` in java 11

The informal Spotless styleguide has been "any style that matters is enforced by spotlessApply". Now that we're compiling against Java 11 world, there's a big new degree of freedom, which is when to use `var` or not.

I'd like to adopt a style which is "you must always use var, unless the line has a comment".

```java
// before spotlessApply
String example = "blah";

// after spotlessApply
var example = "blah";
```

```java
// before spotlessApply
String example = "blah"; // keep type

// after spotlessApply
String example = "blah"; // keep type
```

I'd like it to be a discrete step, ala

```java
spotless { java {
gjf() // or whatever the user is currently using
alwaysUseVarUnlessCommented()
} }
```

But if there's a way to configure an existing formatter, GJF, palantir-java-format, or eclipse, that would work too.

Suggestions and PRs welcome!