`lint`: merge `indent()` and `space()` options into one

[luisesanmartin]

• indent() lets the user select the number of spaces for indentation the linter should check. Default is 4
• space() allows to define the number of spaces indentation hard tabs should be replaced for. Default is the same value for indent().

Having these two inputs in different places adds confusion to the users. It's also probably not very useful anyways: there won't be much cases (or any!) when a user will want the linter to check a certain number of spaces for indentation, but then use a different number when replacing hard tabs with spaces.

We should use only indent() for both functionalities but keep space() usable (without mentioning it in the help file or vignettes) for backwards compatibility.