Add ESLint rules to enforce existing lib style (#621)

Adds a few rules that enforce consistency across the repo.

Only change is one top-level const arrow function is now a function
definition.

src/utils/type.ts

@@ -13,6 +13,6 @@ export function toClass<P, C>(cls: new (plain: P) => C, obj: P | C): C {
 /**
  * Checks if an object is an instance of {@link IColorable}.
  */
-export const isColorable = (obj: unknown): obj is IColorable => {
+export function isColorable(obj: unknown): obj is IColorable {
   return typeof obj === "object" && obj !== null && "setColorOption" in obj && "getColorOption" in obj
 }