Replaces Acorn over Travis CI by ESLint over (Microsoft) GitHub Actions

> See #11, Acorn was great but not sufficient for Mozilla's libPref syntax...

.eslintrc.yml

@@ -0,0 +1,45 @@
+%YAML 1.2
+---
+
+root: true
+
+extends: 'eslint:recommended'
+
+globals:
+  # From <https://searchfox.org/mozilla-central/rev/c938c7416c633639a5c8ce4412be586eefb48005/modules/libpref/parser/src/lib.rs#296>
+  pref: true
+  user_pref: true
+  sticky: true
+  locked: true
+  sticky_pref: true
+
+rules:
+  # Expect a semicolon after each statement.
+  semi:
+    - "error"
+    - "always"
+    -
+      omitLastInOneLineBlock: false
+
+  # As internal code style, don't allow tabulation.
+  no-tabs: "error"
+  # ... nor trailing spaces !
+  no-trailing-spaces: "error"
+
+  # Expect only double-quoted strings.
+  quotes:
+    - "error"
+    - "double"
+
+  # Don't allow whitespace before semicolons.
+  semi-spacing:
+    - "error"
+    -
+      before: false
+
+  # Don't allow irregular whitespace characters in our sheet.
+  no-irregular-whitespace:
+    - "error"
+    -
+      skipStrings: false
+      skipComments: false