[prism] Model inline conditionals as breakables

[reese]

Currently for mod conditionals (like return if foo), we transform those to multiple lines (if foo \n return \n end) if they're multiline/too long. The current implementation does this in a wildly inefficient manner: cloning the ~entire parser state, rendering the whole thing as a string, and then checking if the string is multiline. It does all this every time a mod conditional comes up, so it currently takes up a pretty decent chunk of profiling, not to mention it's kinda a hack.

This PR makes conditionals work like any other breakable. It stashes the tokens for the predicate and the body of the conditional, then uses those to render into either an inline or block form depending on the line length. This should more accurate and efficient (with much less cloning/allocating) than rendering every single time.