feat(config): ui width, auto exec, selection and navigation configs (#80)

closes #67

So I made an auto-exec confg `auto-exec = true`
(e32c0ba6ed69f3c764353e58b8fbacf160f37d5a) which means:
- If you want your command in the prompt to be executed right away, you
can set this = false (for example, you are typing `nvim ~/.conf` but
IRIS is suggesting `nvim ~/.config`, it will ignore the suggestion and
exec what you are typing, this is current default behavior)
- If you want to auto execute full command which is being suggested, you
can set this = true (this is the opposite of above, when you are typing
`nvim ~/.conf` but IRIS is suggesting `nvim ~/.config`, it will auto
exec `nvim ~/.config` instead, but sometime you want to exec what you
are typing instead of the suggestion right?, just press esc or shift tab
to hide the menu and it will)

---

Select keybinding to replace tab
(0dc01cba8c610e8d6fd8c50f6a71bdcc061a098f)
`select = "ctrl+y"`

Some users may love nvim keybinding or emacs, so I bring navigation
keybinding to be configurable (0dc01cba8c610e8d6fd8c50f6a71bdcc061a098f)
```
navigate-up = "ctrl+k"
navigate-down = "ctrl+j"
```
---

I also added a option for UI width
(dfa3b810a7be406fe4652b3886020f37bb166811)
`max-width = 80`
- If you configure `max-width` to be larger than the actual width of the
current terminal, Iris will automatically "compress" the window to match
the terminal's width (meaning it will never overflow and break the
interface)
- If you configure `max-width` too small (e.g 10), the interface will
have cramped text, making it look very bad. So, I've set a minimum safe
min limit of 40. If the configured value is smaller than this, Iris will
automatically use 40

Small note: If you set `max-width = 0` (or leave it blank), it will
automatically use Iris's default standard size of 76. SO you can
confidently customize it without fear of breaking the UI (I hope so
lmao)

---

Added a fallback to ensure Iris never crashes or loses shortcuts if the
user accidentally deletes keybinding configuration lines or
intentionally assigns empty strings " "
(f682c9bf623f3ed46ad0763221145604dbd91b07)

Fix typo `toggle_mode` to `toggle-mode`
(b39be6673e552735fa65a6c9807510c83b4d9cdd)

if you have a better config design, please tell me, I always listen to
your idea

internal/config/defaults.go

@@ -11,6 +11,7 @@ func DefaultConfig() *Config {
 			Mode:        "last",
 			Debug:       false,
 			ExpandAlias: true,
+			AutoExecute: false,
 		},
 		UI: UIConfig{
 			Style:           "modern",
@@ -18,6 +19,7 @@ func DefaultConfig() *Config {
 			ShowHiddenFiles: false,
 			MaxSuggestions:  100,
 			MaxHeight:       15,
+			MaxWidth:        0, // 0 means no limit, fallback to terminal width
 			NerdFonts:       true,
 		},
 		Git: GitConfig{
@@ -42,8 +44,11 @@ func DefaultConfig() *Config {
 			},
 		},
 		Keybindings: KeybindingsConfig{
-			ToggleMode: "ctrl+r",
-			ToggleMenu: "shift+tab",
+			ToggleMode:       "ctrl+r",
+			ToggleMenu:       "shift+tab",
+			SelectSuggestion: "tab",
+			NavigateUp:       "up",
+			NavigateDown:     "down",
 		},
 	}
 }