fix(termdebug): send SIGINT when interrupting prompt mode

Unlike Vim's job_stop(), Nvim's jobstop() does not take a signal argument, and
always sends SIGTERM/KILL.

:Stop and Ctrl-C in prompt mode is supposed to interrupt the program like in
terminal mode, not kill GDB.

Also, maybe libuv's kill() works on Windows? If so, the logic above could be
removed, but I don't have a Windows machine available to test that.

Also "set nomodified" when ending prompt mode, like Vim (avoids E37).

runtime/pack/dist/opt/termdebug/plugin/termdebug.vim

@@ -624,7 +624,7 @@ func s:PromptInterrupt()
       call debugbreak(s:pid)
     endif
   else
-    call jobstop(s:gdbjob)
+    call v:lua.vim.uv.kill(jobpid(s:gdbjob), 'sigint')
   endif
 endfunc
 
@@ -821,6 +821,7 @@ func s:EndPromptDebug(job_id, exit_code, event)
 
   let curwinid = win_getid()
   call win_gotoid(s:gdbwin)
+  set nomodified
   close
   if curwinid != s:gdbwin
     call win_gotoid(curwinid)