Update README.md: add JupyterLab client

[krassowski]

Thank you for maintaining this language server! Just wanted to let you know that we support JavaScript and TypeScript using your package, and add a shameless promotion to help users who may look for a client with JS/TS support.

While we may be late to the party, our repo has a link to binder which allows running the JupyterLab with LSP integration in the browser without installing anything - might be of use to some "searching" users.

[codecov-io]

Codecov Report

Merging #658 into master will not change coverage. The diff coverage is n/a.

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##           master    #658   +/-   ##
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  Coverage    82.4%   82.4%           
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  Files          15      15           
  Lines        2126    2126           
  Branches      448     448           
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  Hits         1752    1752           
  Misses        364     364           
  Partials       10      10

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