feat: add ISO (UK) keyboard layout support

[NakaokaRei]

Overview

Add support for the ISO UK keyboard layout.

The UK layout shares the ISO physical keyboard (extra key next to left Shift) but has its own character mapping that differs from US:

Char	US	UK (ISO)
"	Shift+'	Shift+2
£	N/A	Shift+3
#	Shift+3	\ key (0x0A)
@	Shift+2	Shift+'
~	Shift+`	Shift+` (same)
\	\ (0x2A)	ISO extra key (0x0A)
|	Shift+\	Shift+ISO extra key
`	` (0x32)	` (0x32) (same)

Tasks

• Add .uk case to KeyboardLayout
• Add ISO-specific keys to Key enum (e.g., .isoSection for the extra key, keycode 0x0A)
• Implement UK character mapping in KeyboardLayout
• Update detect() to distinguish ISO keyboards (kKeyboardISO)
• Add tests for UK layout mappings
• Update README

Related

• PR feat: add JIS keyboard layout support #56 (JIS keyboard layout support)