Allow ignore end-of-line inside delimiters

[casey]

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.