[lldb-dap] Add an introductory message on startup. (#170795)
This adds an introductory message to try to inform users on how the
debug console works and adds a little more information on the current
target/process, similiar to the lldb driver.
Here is an example of the introduction:
```
To get started with the debug console try "<variable>", "<lldb-cmd>" or "help [<lldb-cmd>]".
For more information visit https://lldb.llvm.org/use/lldbdap.html#debug-console.
Executable binary set to 'a.out' (arm64-apple-macosx15.0.0).
Attached to process 1234.
```