[lldb-vscode] Creating a new flag for adjusting the behavior of evaluation...

[lldb-vscode] Creating a new flag for adjusting the behavior of evaluation repl expressions to allow users to more easily invoke lldb commands.

This adds a new flag and lldb runtime command to allow users to manage the behavior of the lldb-vscode evaluate repl request.

When evaluating a repl context this now has runtime managed flag for control how the repl behaviors with the follow values:

* `variable` - the existing behavior, with this mode requests are evaluted in the current frame context as variable expressions. To trigger a lldb command prefix an expression with ` and it will be evaluted as an lldb command.
* `command` - all expressions are evaluated as lldb commands.
* `auto` - An alternative mode that will attempt to determine if the expression is an lldb command or a variable expression. Based off the intepreted results the expression will be evaluted either as a command or an expression.

Additionally, I enabled completions and ensured they work with the new repl expression behavior to provide auto-completes.

RFC Discussion: https://discourse.llvm.org/t/rfc-lldb-vscode-evaluate-repl-behavior-and-improvements/71667

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D154030