tools: enforce linting for unix-style line endings

PR-URL: #6685
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>

.eslintrc

@@ -60,6 +60,7 @@ rules:
   indent: [2, 2, {SwitchCase: 1}]
   key-spacing: [2, {mode: "minimum"}]
   keyword-spacing: 2
+  linebreak-style: [2, "unix"]
   max-len: [2, 80, 2]
   new-parens: 2
   no-mixed-spaces-and-tabs: 2