Implement modular scale theme utility function

This commit implements the `ms` shorthand function for polished's (1)
`modularScale` helper with pre-configured base and ratio values.

References:
  (1) https://polished.js.org

Associated epic: #2
GH-64

src/styles/theme/index.js

@@ -20,7 +20,7 @@ import globals from "./globals";
 import media from "./media";
 import motion from "./motion";
 import normalize from "./normalize";
-import { generateMediaQuery, themedMode, themedModeVariant } from "./utils";
+import { generateMediaQuery, ms, themedMode, themedModeVariant } from "./utils";
 import typography from "./typography";
 import { MODE_BRIGHT_SNOW_FLURRY, MODE_DARK_NIGHT_FROST, THEME_KEY_MODE } from "./constants";
 
@@ -32,6 +32,7 @@ export {
   globals,
   media,
   motion,
+  ms,
   nord,
   normalize,
   palettes,

src/styles/theme/utils/index.js

@@ -15,7 +15,8 @@
  */
 
 import generateMediaQuery from "./generateMediaQuery";
+import ms from "./ms";
 import themedMode from "./themedMode";
 import themedModeVariant from "./themedModeVariant";
 
-export { generateMediaQuery, themedMode, themedModeVariant };
+export { generateMediaQuery, ms, themedMode, themedModeVariant };

src/styles/theme/utils/ms.js

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+import { modularScale } from "polished";
+
+import typography from "../typography";
+
+/**
+ * Shorthand function for polished's `modularScale` helper with pre-configured base and ratio values.
+ *
+ * @method ms
+ * @param {number} step The step to scale up or down.
+ * @return {string} The calulcated scale with the `em` unit.
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @see https://polished.js.org/docs/#modularscale
+ * @since 0.3.0
+ */
+const ms = step => modularScale(step, typography.sizes.msBase, typography.sizes.msName);
+
+export default ms;