fix: ensure supportsColor.level exists before stringifying it

When supports-color detects a level of 0, it will not return an object containing the key `level`, but instead only `false`. This commit makes sure `supportsColor.level` exists before calling `supportsColor.level.toString()`.

The TypeScript definitions of `chalk` seem wrong in this regard, since they imply that if `supportsColor` is truthy, it is an object and always contains the `level` key of type `0 | 1 | 2 | 3`.

bin/lint-staged.js

@@ -4,7 +4,7 @@
 
 // Force colors for packages that depend on https://www.npmjs.com/package/supports-color
 const { supportsColor } = require('chalk')
-if (supportsColor) {
+if (supportsColor && supportsColor.level) {
   process.env.FORCE_COLOR = supportsColor.level.toString()
 }