Optionally indent with tabs

[lf-]

Describe the option you'd like to add

For accessibility reasons (https://www.reddit.com/r/javascript/comments/c8drjo/nobody_talks_about_the_real_reason_to_use_tabs/), it is useful to use tabs for indentation as they are then configurable in the editor. It appears that Fourmolu currently doesn't support using tabs for indentation at all. For what it's worth, there is currently a possible movement to change the default in prettier to use tabs: prettier/prettier#7475

I personally have to use a rainbow indent guides plugin for 2-space indented code as I can't visually distinguish between levels, which helps marginally, but it would be ideal to have the ability to change the width in my editor.

indent-style: tab
indent-style: space

bikeshed: indentation: tab might be another way of writing this

• What option setting would keep current behavior?
  indent-style: space

Show a code example exemplifying the different option settings

module Mod where

-- indent-style: tabs
sayHello = do
	putStrLn "hello"
	putStrLn "world"

-- indent-style: spaces (with indentation: 2)
sayGoodbye = do
  putStrLn "goodbye"
  putStrLn "world"

[georgefst]

bikeshed: indentation: tab might be another way of writing this

Seems preferable to me, in the usual vein of "make illegal states unrepresentable".