Add icons for each diagnostic in summary

[RomaLzhih]

When we summarize the diagnostic using :lspsaga show_workspace_diagnostics, the diagnostics are listed line by line; the only difference between them is the color, which is not friendly for color-blind users.

In this case, it would be much more convenient to add a diagnostic icon showing the severity of them for difference, i.e.,: a bug icon in front of the error diagnostic