Add ability to load Language Servers as extensions (bundles)

[pkasila]

Description

Now ExtensionsManager supports language_server extensions. Also, there is a LSPClient class added to handle Language Server Process (through stdio). Currently, it doesn't do anything except running Language Server in the background.

Language Server Extension Bundle

Almost the same as in #416, but there are some minor changes in the process.

1. Plugin's SDK should be set to language_server while registering a plugin in the Extension Store
2. Extension should contain Language Server executable inside
3. Extension's Info.plist should contain the following properties
   3.1. CELSPExecutable (String) which is the Language Server's executable file name
   3.2. CELSPFileExtensions (Array) which contains file extensions to be handled by Language Server
   3.3. CELSPArguments (Array, optional: defaults to ["--stdio"]) which contains arguments to be passed to Language Server to be run

Language Server Extension example is available in the pkasila/HTMLLanguageServer repository (based on vscode-html-languageserver)

Related Issue

• Fix Marked Text Input CodeEditTextView#40
• 🐞 Threading Issues #86
• ✨ Extension Store #183

Checklist

• I read and understood the contributing guide as well as the code of conduct
• My changes generate no new warnings
• My code builds and runs on my machine
• I documented my code
• Review requested

[austincondiff]

Outstanding, love it!