Autocomplete

[cosmos72]

implement code autocompletion (feature request #22) by adapting @jpark-dev https://github.com/jpark-dev/gophernotes/tree/feature/autocomplete_new_gomacro to use gomacro/fast/Interp.CompleteWords()

Features:

• Side-effects free: does not use gomacro/fast/Interp.RunExpr()
• Works across multiple code cells
• Not confused by code after the cursor
• Can recursively expand keywords, global symbols (constants, functions, types, variables), content of imported packages, method names and field names

Limitations:

• No completion on local variables
• No completion on the result of complicated expressions (function calls, indexing...)