Allow JavaTemplate to throw new Exception

[timtebeek]

What's changed?

Update BlockStatementTemplateGenerator to separately handle J.MethodInvocation, and not produce an assignment when the methodInvocation has a void return type. This prevents a compiler error for such templates. Additionally wrap any generated template for J.MethodInvocation in a block (class Template {{ ... }}, as opposed to class Template { ... }) to allow for throwing exceptions.

What's your motivation?

Broke the recipes generated for this one, which contained final JavaTemplate after = JavaTemplate.builder("throw new AssertionError();").build();.
https://github.com/PicnicSupermarket/error-prone-support/blob/d81fe19836db9f0001fb9c01370a3b808c46b440/error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/TestNGToAssertJRules.java#L156-L167

Have you considered any alternatives or workarounds?

Could have nested the generated template in the j instanceof Expression conditional just below it, but this seemed cleaner.

Any additional context

As part of the work done towards

• Add recipes to migrate from TestNG to AssertJ rewrite-testing-frameworks#520