Make distinction when removing leading whitespace from trailing whitespace

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> Hi, I've noticed that there's only the `files.trimTrailingWhitespace` without a `files.trimLeadingWhitespace` counterpart. This has been a problem for editing files that mix markdown documentation with another syntax for a while. For example, Elixir files can have markdown documentation using the `@doc` attribute. However, If I enable the trim trailing whitespace option, this removes both trailing and leading whitespace on the lines, affecting markdown line breaks: ```elixir @doc """ My function documentation markdown. This will be part of the first line because the "line break" was removed when saved """ def hello(), do: :world ``` This has been a constant headache because we enforce the removal of unnecessary leading whitespaces from source code, but we can't use the options to remove it all, thus affecting our documentation. If possible, I'd like to be able to remove only leading or trailing whitespace (for this particular case, removing only leading whitespace so markdown docs don't get butchered). **Update 06/03/2021:** After discussing this issue in-depth in [this forum thread](https://elixirforum.com/t/elixir-file-formatting-with-vscode-removing-whitespaces-and-documentation/37966/13), I noticed that perhaps the phrasing/ semantics of the issue is not perfect enough. I don't know for sure what would be the correct solution, but I hope that I could make the problem we are facing relatable. So, thanks for everyone that helped so far!