[lldb-vscode] Allow specifying a custom escape prefix for LLDB commands (#69238)

We've been using the backtick as our escape character, however that
leads to a weird experience on VS Code, because on most hosts, as soon
as you type the backtick on VS Code, the IDE will introduce another
backtick. As changing the default escape character might be out of
question because other plugins might rely on it, we can instead
introduce an option to change this variable upon lldb-vscode
initialization.
FWIW, my users will be using : instead ot the backtick.