feat: add custom format-icons for each key in keyboard-state module

[Cprakhar]

Description

This PR adds new options to specify icons for different keys (e.g., "numlock", "scrolllock", "capslock").
Closes: #4848

Example:

"keyboard-state": {
        "format": "{name} {icon}",
        "format-icons": {
            "capslock": {
                "locked": "CL",
                "unlocked": "cl"
            },
            "numlock": {
                "locked": "NL",
                "unlocked": "nl"
            },
            "scrolllock": {
                "locked": "SL",
                "unlocked": "sl"
            }
        },
        "numlock": true,
        "scrolllock": true,
        "capslock": true
    }

Capslock: OFF

Capslock: ON

It is backward-compatible; if locked or unlocked is defined as below, then it uses the same icons for all keys.

"keyboard-state": {
        "format": "{name} {icon}",
        "format-icons": {
           "locked": "",
           "unlocked": ""
        },
        "numlock": true,
        "scrolllock": true,
        "capslock": true
    }