Move metals-vscode to a subdirectory

[gabro]

This PR prepares the ground for folding https://github.com/scalameta/metals-languageclient back into this repo.
The general idea is that https://github.com/scalameta/metals-languageclient used to be used by coc-scala and metals-vscode, but since coc-scala is now deprecated it doesn't make much sense to split the common code to a separate repository any longer.

Still, I think it makes sense to keep metals-languageclient as a separate package for two reasons:

• in the odd chance something was depending on it, it can keep doing so
• it may be useful to someone writing js-based Metals clients in the future

Now, to achieve the goal, I would normally use a yarn workspace, which would look like this:

├── package.json
├── yarn.lock
├── packages
│   ├── metals-vscode
│   │   ├── package.json
│   │   ├── ...
│   └── metals-languageclient
│       ├── package.json
│       ├── ...
└── node_modules

However, due to vsce not supporting yarn workspaces we cannot have a proper workspace, so each subdirectory will live on its own and we don't get features like symlinking or dependency hoisting and result in something like:

├── packages
│   ├── metals-vscode
│   │   ├── package.json
│   │   ├── yarn.lock
│   │   ├── node_modules
│   │   ├── ...
│   └── metals-languageclient
│       ├── package.json
│       ├── yarn.lock
│       ├── node_modules
│       ├── ...

I think this is mostly fine; it will require some fiddling, but nothing too crazy.
And if microsoft/vscode-vsce#300 ever gets merged, we can then easily move to a proper workspace.

Once this PR lands, we can then add the contents of the metals-languageclient repo as a directory next to metals-vscode. In order to do that preserving the git history, I will need to rewrite the repo history, so it would be better to cleanup the PR queue before that.

[kpodsiad]

LGTM

I will need to rewrite the repo history, so it would be better to cleanup the PR queue before that.

I think there are no important PRs waiting tio be merged