chore: upgrade prettier to 2.4.1

Prettier 1.19 (the version we used before this MR) didn't support the TS version we use. It only became apparent when we needed to use the latest syntax to make esbuild work properly as a part of #453 (closed)

All the changes in this MR are automatically generated by running npm run autofix (which runs prettier --write) except for:

• package.json and package-lock.json which were updated by running npm install --save-dev prettier@latest
• .prettierrc - I had to add rule "arrowParens": "avoid" to avoid re-formatting hundred files with different arrow function style
• src/webview/.eslintrc.json - I had to turn off the vue/singleline-html-element-content-newline because it was clashing with prettier and prettier can't be configured

The prettier doesn't offer more granular rules. All the changes in this MR are necessary for the upgrade.

Related to #453 (closed)