refactor: use toString, expect_named, lengths, expect_null, fixed in grepl

[m-muecke]

• Use testthat::expect_match()
• Use testthat::expect_named()
• Use testthat::expect_null()
• Use toString(x)
• Use lengths(x)
• Use fixed for grep where possible
• Updated .lintr config
• Missing docs
• Add missing explicit integers