Add dictionary features

[Crivella]

If requested by the extension, server should be able to provide definitions.
The server should return a per-word or per-block definition that the extension will than add as a popup or tooltip to the text

• Add an option to turn this behavior on/off
• Server should communicate list of available dictionaries (eg list of languages that support this feature)
• As dictionaries might be bulkier, they should be supplied separately (eg specific repos or even on a plugin basis)
  • Automatically downloaded when requested
  • And/or tool to allow user to manage installed/downloaded dictionaries