Create a plugin system

[MilesCranmer]

As discussed on slack.

Here is the accompanying BorrowChecker.jl extension, which I have manually tested in VSCode: MilesCranmer/BorrowChecker.jl#42

For example, here is my .vscode/settings.json for this to work:

{
  "jetls-client.executable": [
    "julia",
    "--startup-file=no",
    "--history-file=no",
    "--project=/Users/mcranmer/PermaDocuments/BorrowChecker.jl/worktrees/jetls2/JETLS.jl",
    "-m",
    "JETLS"
  ],
  "jetls-client.settings": {
    "plugins": [
      {
        "name": "BorrowChecker",
        "uuid": "7bdcaa52-c310-4bb0-bf54-d941056ed284",
        "path": "..",
        "enabled": true
      }
    ]
  }
}