Add lsp configuration support

[azeier]

This allows for user-level lsp configuration via zed/settings.json.

I have personally just disabled scss-lsp entirely, but this provides an additional workaround for #23 while keeping both enabled via

"lsp": {
  "some-sass-lsp": {
    "settings": {
      "somesass": {
        "scss": {
          "completion": {
            "enabled": false
          }
        }
      }
    }
  },
  "scss-lsp": {
    "settings": {
      "scss": {
        "hover": {
          "documentation": false,
          "references": false
        }
      }
    }
  }
},

(It seems that it's not possible to disable scss-lsp completion)

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In action with the above and some-sass colors enabled:

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Full some-sass configuration options can be found here: https://wkillerud.github.io/some-sass/user-guide/settings.html#scss
Full scss-lsp configuration options can be found here: https://code.visualstudio.com/docs/languages/css#_customizing-css-scss-and-less-settings