[lldb-vscode] add `launchCommands` to handle launch specific commands

Summary:
This can help `lldb-vscode` handle launch commands associate with remote platform
attach request have field `attachCommands` to handle attach specific commands
add a corresponding one for launch request
if no launch command is provided, create a new target and launch; otherwise, execute the launch command

Differential Revision: https://reviews.llvm.org/D65363

Patch by Wanyi Ye <kusmour@gmail.com>

llvm-svn: 369296