Add eslint rule to ignore underscore in hooks

[ohhheyyyy]

Removes the ESLint error created and discussed at approximately 2:52 in the Context Q&A video under the Special Case React Tools section of the Complete Intro to React, v8 Frontend Masters course.

This PR adds:

"no-unused-vars": ["error", {
  "varsIgnorePattern": "^_"
}]

to the .eslintrc.json "rules" object.

which allows us to remove:

// eslint-disable-next-line no-unused-vars

from line 15 of the 14-context/src/Details.jsx file.