Fix incorrect line numbers with some IF blocks

If a line ended with a string's closing quote, or a newline escape, then
skipping over that line via IF/ELIF/ELSE would fail to count that line,
offsetting the rest of the file.
I have no idea why but for some reason 9829be1 changed *specifically*
`if_skip_to_else` to have incorrect behavior on string endings. The incorrect
behavior on newline escapes seems to have been here since the beginning.
Also added a test to check for both of those behaviors in both functions.

Honestly, it baffles me that nobody ever noticed. I didn't until I started
working on #395.

test/asm/correct-line-number.asm

@@ -0,0 +1,11 @@
+IF 0
+	"\
+	"
+ELIF 1
+	WARN "Am I geting ahead of myself?"
+ELSE
+	"\
+	"
+ENDC
+
+WARN "Hopefully not."