Introduce `exact_line_breaks(count)` IR builder.
This produces a line element that prints exactly `count` line breaks, exempt from the printer's hardline collapsing / empty-line capping.
Blank runs inside verbatim values (YAML block/flow scalars, GraphQL block strings) are the value itself and must not be normalized.
Previously this was emulated by emitting raw `\n` text followed by a re-arming `hard_line_break()`..., hack.
Also removed the printer behavior that trims end of line whitespaces.
leaysgur