Add option to preserve newline gaps for blocks

[InoUno]

Adds an option that allows preservation of newline gaps at the start and/or end of blocks - see example at the end of PR.

This is similar to how newline gaps are already preserved between statements. Namely that the existence of newline gaps between statements is also dependent on if there's a gap in the input string:

local x = 1 -- the below gap is preserved

local y = 2

I'm aware that there's discussion about if more config should be added in #620, so this PR can serve as another option suggestion.

Adding this option solves this issue, but in a more general way for all blocks, instead of just for if-else-if chains.

This could instead be implemented on a subset of newline gaps (i.e. only for leading newline gaps before else-if/else tokens), or even enforced always with a different option, as suggested in the above issue.
However, I went with the more general solution in this PR, since notably gofmt also preserves newline gaps at the start and end of blocks in this way as well. It is also a quite opiniated formatter, but doesn't have an opinion on those gaps (similar to statement gaps), besides them being at most a 1-newline gap.

Motivation and example

In our codebase, we occasionally have some long if-else-if chains, and they become quite hard to read when there's no newline gaps allowed in them. This is especially true when the condition itself is multiline, since it then becomes indented at the same level as the inner block statements:

if
    this_is == very_long_variable_name
    and to_ensure_that == it_is_broken_into
    and multiple_lines == in_order_to_see_how_that_looks
then
    do_this_thing = 0
elseif
    this_is == very_long_variable_name
    and to_ensure_that == it_is_broken_into
    and multiple_lines == in_order_to_see_how_that_looks
then
    do_something_here = 1
    with_a_few_long_lines_as_well = 2
elseif
    this_is == very_long_variable_name
    and to_ensure_that == it_is_broken_into
    and multiple_lines == in_order_to_see_how_that_looks
then
    do_something_else_here = 3
    with_a_few_long_lines_as_well = 4
else
    return 1
end

Having newline gaps makes it a bit easier to distinguish where the different if-blocks start and end:

if
    this_is == very_long_variable_name
    and to_ensure_that == it_is_broken_into
    and multiple_lines == in_order_to_see_how_that_looks
then
    do_this_thing = 0

elseif
    this_is == very_long_variable_name
    and to_ensure_that == it_is_broken_into
    and multiple_lines == in_order_to_see_how_that_looks
then
    do_something_here = 1
    with_a_few_long_lines_as_well = 2

elseif
    this_is == very_long_variable_name
    and to_ensure_that == it_is_broken_into
    and multiple_lines == in_order_to_see_how_that_looks
then
    do_something_else_here = 3
    with_a_few_long_lines_as_well = 4

else
    return 1
end