Add formatting support for VSCode logpoints message

https://reviews.llvm.org/D127702 adds the initial logpoints support in
lldb-vscode. This patch further improves it by:
1. Adding a newline at the end of each log message
2. Support most of the format specifiers like \t, \n, \x etc..

The implementation is borrowed from FormatEntity::ParseInternal(). Future
patch should merge these two implementations.

Differential Revision: https://reviews.llvm.org/D136697