Modify the lexer to keep track of opening `({[` and closing `]})` delimiters.
When the lexer would emit an eol or indent outside of a recipe when there
is at least one open delimiter, emit a whitespace token instead.
This allows expressions to be split on multiple lines, like so:
x := if 'a' == 'b' {
'x'
} else {
'y'
}
This does not work inside of recipe body interpolations, although this
restriction might relaxed in the future.
