Add Input mode for collapse_simple_statement

[ddashdev]

Adds a new Input mode for collapse_simple_statement.

It allows Stylua to preserve whether simple functions and conditionals were originally collapsed or expanded, instead of enforcing one global collapse behavior across the whole codebase.

With this mode:

if x then return end

stays collapsed, while:

if x then
    return
end

stays expanded.

The motivation is making it possible for users to keep a mixed style where some simple statements are collapsed and others are intentionally expanded, not all simple statements carry the same importance in a codebase. In some cases a collapsed statement improves readability by keeping a guard clause or empty function compact, while in others, expanding the statement makes the control flow or intent more visible.

The existing collapse_simple_statement modes require users to choose one global behavior: collapse matching statements everywhere, or expand them everywhere. This can be limiting when different simple statements benefit from different formatting choices.

And, while stylua: ignore comments can work around individual cases, they add noise and become difficult to maintain when used repeatedly for formatting preferences. Adding an Input mode follows the existing input-preserving semantics used by other Stylua rules like call_parenthesis, and gives users the ability to preserve these choices per situation without changing the default behavior.

Note: I used AI assistance while working through parts of the implementation, but reviewed the resulting changes and verified the behavior with tests.

Closes #863

[ddashdev]

Also closes #863

[d-enk]

Thanks for working on this — I'm the author of #863, and this addresses what I was hoping for.

One gap though: the mode currently only preserves the input shape for functions and if-guards, but regular block statements (do/for/while) still get expanded even when they were collapsed in the input. For example, this idiom (common in UTF-8 decoders and similar tables) currently expands into 12 lines:

-- input
for i = 0x00, 0x7F do sequence_len_by_byte[i] = 1 end
for i = 0xC0, 0xDF do sequence_len_by_byte[i] = 2 end
for i = 0xE0, 0xEF do sequence_len_by_byte[i] = 3 end
for i = 0xF0, 0xFF do sequence_len_by_byte[i] = 4 end

The same applies to do local x = 1 end and while cond do step() end.

The approach could extend naturally to all block-bearing statements: check whether the whole statement spans a single line in the input and the block is "simple" (a single laststmt / simple stmt, no comments), then inline the block instead of deferring to format_block().

[JohnnyMorganz]

Thank you!