caddyfile: Prevent bad block opening tokens

[francislavoie]

Fix #4501

Basically, we want to prevent two cases where bad block openings can happen.

localhost
reverse_proxy { to abc:123 }

☝️ this one used to result in the upstreams list containing } (the last token on the line) because since no newline happens, to reads all the remaining tokens on the same line, consuming both abc:123 and }, but never closing the block.

This bug is being fixed by checking if after we see a { token, whether the next token is on a new line; if not, err. This could break "legitimate" situations where some hypothetical expression-like directive wants to consume all the tokens on the line where { is some syntactical token of that "language", but I think this is unlikely, and can be worked around by quoting the tokens like "{" and "}" (or with backticks).

localhost
reverse_proxy abc:123 {}

☝️ this one ends up in an upstreams list with abc:123 and {}; I don't think there's ever a case where there'd be a placeholder with no name, and having it at the end of line seems like almost always a mistake (where the user is trying to put an empty block in their config).

Same idea, we're checking if when we see {} whether the next token is on a new line, and if so, err. This can also be worked around by quoting with "{}"

Worth noting that #4643 made the quote workaround possible in this implementation, because now we know whether a particular token was quoted. We would only know the token value previously, without context of quoting. 🎉

[mholt]

Thanks Francis, this is cool. We can give this a try!