Add autocomplete for Solidity

[samajammin]

Summary

Add autocomplete functionality similar to how the Solidity plugin for VSCode (which also uses Monaco):

Currently this works for variable names:

But not for built-in values, like msg.sender:

Motivation

This would be a nice DX improvement.

Describe alternatives you've considered

None.

Additional context

Recommendation from a recent feedback survey of Ethereum developers.