Introduce Checkstyle rule for separator symbol location

[sbrannen]

We have a longstanding rule in Spring Framework about where separator symbols are allowed to be located in the code.

Specifically, separator symbols must always come at the end of the line, not at the beginning of the next line.

This is documented in our wiki, but the rule is not currently enforced.

Place separator symbols at the end of the current line rather than at the beginning of the next line. In this context, the following are considered separator symbols: ,, +, ?, :, &&, ||.

We should therefore introduce a Checkstyle rule to enforce this.

[sbrannen]

Closed via 208d52d and 92ee20c.