Add support for textEdit

[axelson]

Related issue: #27

insertText is deprecated because it is too open to interpretation and not precise enough. Instead we should support textEdit

• insertText deprecation: https://github.com/microsoft/language-server-protocol/blame/56e4fc292c121118ac520c8a19688c85066ed0bb/specification.md#L2946
• textEdit details: https://github.com/microsoft/language-server-protocol/blame/56e4fc292c121118ac520c8a19688c85066ed0bb/specification.md#L434

[lukaszsamson]

As of 3.15 (and upcoming 3.16) insertText is no longer deprecated. The spec now only recommends using texEdit and a lot of other are being introduced to this APIs

[axelson]

Good point, updated the title