workaround for source-map crash on newer node.js

while the previous commit reminds devs to `nvm use` the
officially-supported version of node.js this dependency workaround helps
resolve an unneccessary gatsby crash on newer versions in the future

see
gatsbyjs/gatsby#35607 (comment)
for background

package.json

@@ -98,6 +98,9 @@
  "prettier": "2.2.1",
  "tiny-glob": "^0.2.8"
  },
+ "resolutions": {
+ "source-map": "^0.8.0-beta.0"
+ },
  "license": "MIT",
  "scripts": {
  "build": "gatsby build",