improve config error messages for duplicate binds

[Ind-E]

For config files with duplicate keybinds, an additional error is emitted that points to the previous instance of the bind. If a single bind is repeated 3+ times, only the first duplicate will get an additional error.

For example, with this config:

binds {
    Mod+Shift+Slash { show-hotkey-overlay; }

    Mod+Shift+Slash { show-hotkey-overlay; }

    Mod+Tab { focus-workspace-previous; }

    Mod+Tab { focus-workspace-previous; }

    // ...

   Mod+Shift+Slash { show-hotkey-overlay; }
}

This error is shown

Error:   × error loading config
  ├─▶ error parsing
  ╰─▶ error parsing KDL

Error:   × duplicate keybind first defined here
     ╭─[config.kdl:337:1]
 337 │ binds {
 338 │     Mod+Shift+Slash { show-hotkey-overlay; }
     ·     ───────┬───────
     ·            ╰── node starts here
 339 │
     ╰────
Error:   × duplicate keybind later defined here
     ╭─[config.kdl:339:1]
 339 │
 340 │     Mod+Shift+Slash { show-hotkey-overlay; }
     ·     ───────┬───────
     ·            ╰── unexpected keybind
 341 │
     ╰────
Error:   × duplicate keybind first defined here
     ╭─[config.kdl:341:1]
 341 │
 342 │     Mod+Tab { focus-workspace-previous; }
     ·     ───┬───
     ·        ╰── node starts here
 343 │
     ╰────
Error:   × duplicate keybind later defined here
     ╭─[config.kdl:343:1]
 343 │
 344 │     Mod+Tab { focus-workspace-previous; }
     ·     ───┬───
     ·        ╰── unexpected keybind
 345 │
     ╰────
Error:   × duplicate keybind later defined here
     ╭─[config.kdl:600:1]
 600 │
 601 │     Mod+Shift+Slash { show-hotkey-overlay; }
     ·     ───────┬───────
     ·            ╰── unexpected keybind
 602 │ }
     ╰────