Rule request: Enforce only tabs OR spaces

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

As this is in my eyes a pretty obvious thing, please just close if there's an existing rule for this. I searched extensively and didn't find a rule or existing issue.

As far as I can tell, the indentation_width rule only enforces use of consistent amount of spaces or tabs. However, depending on an editor's set tab width, this can lead to inconsistent indentation when one developer uses tabs, another spaces. So SwiftLint should enforce that only one style of indentation is used (and preferably enforce tabs, of course 😬).

Should trigger warning (as GitHub uses a tab width of eight, it's even easy to see here):

private func test() {
	print("One Tab")
    print("Four Spaces")
}

Current output:

$ swiftlint --version
0.46.2
$ swiftlint --enable-all-rules test.swift
Linting Swift files at paths test.swift
Linting 'test.swift' (1/1)
Done linting! Found 0 violations, 0 serious in 1 file.

I think this should be opt-out, by default it should warn when mixing tabs and spaces, and one should be able to configure what shall be used something like this:

indentation_style: tabs   -> warn when indenting with spaces
indentation_style: spaces -> warn when indenting with tabs

Start by reviewing SwiftLint's existing indentation_width rule and reproduce the behavior with the test.swift example using the shown swiftlint command. Define how indentation_style configuration should select tabs or spaces, then add coverage for mixed indentation and verify the resulting warnings and default behavior.