get_code to return `character(1)` instead of `character(n)`

[gogonzo]

• get_code should return single string with all @code concatenated to a single string.

Please make relevant changes:

• We don't need to use paste(get_code(...), collapse = "\n") everywhere anymore. Please locate all paste(collapse = "\n") in teal.modules.xyz and in teal where code is concatenated to a single string.

[chlebowa]

The internal function format_expression can also be removed. It is used in very few places and that number will soon drop further.

[vedhav]

Maybe I'm missing something. Is there some reason that this issue is bucked into the wider refactor scope? I think it's fine to merge this directly into main without going through refactor.