Minor bug fixes

• Take the indentation of code blocks from the first non-blank line of code in the block. Once we have an indent rule, we can be much more opinionated than this, but too many of the example grammars have hard-to-predict block indentation at the moment.
• Make sure that when looking for return in semantic action bodies, it is at least a whole, single word. The current approach is not good, since we're not parsing the JS inside the block to ensure that the word "return" isn't in a comment or something, but it's better than nothing to catch a common mistake.