bump/improve solhint + prettier

[TateB]

changes:

• bumped solhint to v6 (fixes Update solhint to new Major version 6.0.0 #481)
• bumped prettier-plugin-solidity to v2
• resolved issues with prettier worker
  • i don't really understand how this was working previously, prettier.format returns a promise which wasn't before handled
• added ability to specify the solhint package directory, which will override using the bundled version
• added solhint plugin support, works with solhint-plugin-prettier
• added onDidSave hook, using the same logic as onDidChangeContent
  • without this, if using a different vscode extension for saving (e.g. prettier-vscode), the validation state doesn't seem to update

[juanfranblanco]

Hi, just seen this. I'll review it.