fix(excmd): use realtime for v:starttime, :uptime #39425

Problem:
`v:starttime`, `:uptime` use a monotonic high-resolution timer. This
only works as long as the timer keeps running (if the computer is
suspended the timer is paused). This is somewhat unintuitive, and
doesn't match the behavior of the `uptime` shell command.

Solution:
Implement `os_realtime` to get the real time since the
epoch in nanoseconds.

runtime/lua/vim/_core/ex_cmd.lua

@@ -259,7 +259,9 @@ function M.ex_terminal(eap, shell_argv)
 end
 
 function M.ex_uptime()
-  local uptime = math.floor((uv.hrtime() - vim.v.starttime) / 1e9)
+  -- os.time() might lead to uptime == -1 when this is called too quickly after startup
+  local now = assert(uv.clock_gettime('realtime'))
+  local uptime = math.floor((now.sec * 1e9 + now.nsec - vim.v.starttime) / 1e9)
   local uptime_display = time.fmt_rtime(uptime)
   api.nvim_echo({ { N_('Up %s'):format(uptime_display) } }, true, {})
 end