Add .prettierrc file

[tguichaoua]

This request is not for the library itself, but for the workspace environment.

Is your feature request related to a problem? Please describe.
When I use VS Code, because of there is not .prettierrc file, the prettier pluggin use my default settings.

Describe the ideal solution
Add a .prettierrc file, so the VSC plugin can use the correct settings to format files.

Based on the command:

discord.js/package.json

Line 25 in e0e2711

"prettier": "prettier --write --single-quote --print-width 120 --trailing-comma all --end-of-line lf --arrow-parens avoid src/**/*.js typings/**/*.ts",

.prettierrc

{
  "singleQuote": true,
  "printWidth": 120,
  "trailingComma": "all",
  "endOfLine": "lf",
  "arrowParens": "avoid"
}

package.json

"prettier": "prettier --config .prettierrc --write src/**/*.js typings/**/*.ts",