feat: add Spanish (ISO) keyboard layout support

[NakaokaRei]

Overview

Add support for the Spanish keyboard layout (ISO physical keyboard).

Major differences from US:

Char	US	Spanish
`;`	`;` (0x29)	Shift+`,`
`:`	Shift+`;`	Shift+`.`
`'`	`'` (0x27)	`'` key (different position)
`@`	Shift+2	Option+2
`#`	Shift+3	Option+3
`[`	`[` (0x21)	Option+`[`
`]`	`]` (0x1E)	Option+`]`
`{`	Shift+`[`	Option+`{`
`}`	Shift+`]`	Option+`}`
`ñ`	N/A	Dedicated key (where `;` is on US)

Notes

• Has dedicated key for ñ
• Many symbols require Option key
• Accent characters (á, é, í, ó, ú) via dead key combinations

Tasks

• Add .spanish case to KeyboardLayout
• Handle Option key modifier in CharacterKeyMapping
• Implement Spanish 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, feat: add French (AZERTY) keyboard layout support #59 (Other layouts needing Option key support)