Move some of the dev dependencies to the top level

[jtpio]

Description

Some of the devDependencies seem to be defined in each of the packages:

gator/packages/labextension/package.json

Lines 63 to 69 in bb796be

	"@typescript-eslint/eslint-plugin": "^2.21.0",
	"@typescript-eslint/parser": "^2.21.0",
	"eslint": "^6.8.0",
	"eslint-config-prettier": "^6.10.0",
	"eslint-plugin-prettier": "^3.1.2",
	"eslint-plugin-react": "^7.18.3",
	"husky": "^0.14.0",

We could probably move them to the top-level package.json:

gator/package.json

Line 23 in bb796be

"devDependencies": {

As well as some of the prettier files.