Tidy and expand v3/melgeek/mj6xy_rev3

[dunk2k]

Description

• Add Stepped Caps Lock label and keys
• Reduced left padding
• Add shift row and bottom row layouts to keymap and labels
• Rename and/or re-position labels to accommodate changes
• add layouts.presets

QMK Pull Request

Checklist

• The VIA support for this keyboard is MERGED in QMK master already (MANDATORY)
• The VIA definition follows the guide here: https://caniusevia.com/docs/layouts
• I have a V3 JSON version for this keyboard definition.(MANDATORY)
• I have tested this keyboard definition using VIA's "Design" tab.
• I have tested this keyboard definition with firmware on a device.
• I have assigned alpha keys and modifier keys with the correct colors.
• The Vendor ID is not 0xFEED

[dunk2k]

superseded by #2170