Add agar mini files

[D00mch]

Description

Adds VIA keyboard definitions for two KBDFans Agar Mini variants:

• KBDFans Agar Mini
  • Vendor ID: 0x9D5B
  • Product ID: 0x2507
  • V2 and V3 JSON definitions
  • QMK RGBLight support
• KBDFans Agar Mini EC
  • Vendor ID: 0x9D5B
  • Product ID: 0x2509
  • V2 and V3 JSON definitions
  • Includes the EC Sensitivity layout option used by the VIA app EC Tools pane

Related VIA app PR for the Agar Mini EC read-only ADC tooling:
the-via/app#414

QMK Pull Request

TODO: Add QMK PR link showing VIA support is merged in QMK master.

VIA Keymap Pull Request

TODO: Add VIA userspace keymap PR link:
https://github.com/the-via/qmk_userspace_via/pulls

Checklist

• The VIA support for this keyboard is MERGED in QMK master already (MANDATORY)
• VIA keymap is MERGED in VIA userspace 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 formatted the JSON file to have consistent formatting with the rest of the repository.
• 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