refactor(bad_usb): make HID keyboard layouts table-driven

[yajatnarayan]

Description

Replace the per-layout type_string_us/_abnt2 switch functions with pure data tables and a single generic walker. Each layout is a 128-entry ASCII table of {keycode, modifier, dead_keycode, dead_modifier} entries plus an optional UTF-8 supplemental table; dead keys (ABNT2 accents, '/") are encoded as an optional prefix press. Dispatch is now a singles_layouts[] lookup indexed by ducky_layout_t, so adding a layout needs no new code path.

Output for US and ABNT2 is unchanged. Update the component README to document the new model and the "register a layout" steps.

Related Issue: Fixes #64

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

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Checklist

• I have read the CONTRIBUTING.md document.
• [X My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings during build.
• I have tested this change on physical hardware. (Please specify which target below)

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Hardware Testing Details

N/A