feat: add French (AZERTY) keyboard layout support

[NakaokaRei]

Overview

Add support for the French AZERTY keyboard layout (ISO physical keyboard).

Major differences from US QWERTY:

Char	US	French
a / q	Swapped (AZERTY)
w / z	Swapped (AZERTY)
m	`m` position	Right of `l` (where `;` is on US)
Numbers 0-9	Direct input	Shift required (unshifted = àéè etc.)
@	Shift+2	Option+à (0x00)
[	`[` (0x21)	Option+Shift+(
]	`]` (0x1E)	Option+Shift+)
{	Shift+`[`	Option+(
}	Shift+`]`	Option+)

Notes

• AZERTY has a fundamentally different letter arrangement (A/Q and W/Z swapped, M moved)
• Numbers require Shift to type (unshifted positions produce accented characters)
• Many symbols require Option key
• Has dedicated keys for accented characters (é, è, à, ù)

Tasks

• Add .french case to KeyboardLayout
• Handle Option key modifier in CharacterKeyMapping
• Implement French character mapping
• Add tests
• Update README

Related

• PR feat: add JIS keyboard layout support #56 (JIS keyboard layout support)
• feat: add German (QWERTZ) keyboard layout support #58 (German layout - also needs Option key support)