refactor(Kconfig): Extracted designer defaults out into new files

[Nick-Munnich]

This is an alternative approach to #1886, with the same end goal. All of the "designer configurable" defaults are extracted out to separate files named Kconfig.defaults. The definitions stay in the same Kconfig file, only the defaults move. The defaults are imported after the keyboard config options, allowing them to be overridden by designers.

I used my own judgement to decide whether an option should be treated as "designer configurable". In general:

• Anything related to behaviors or how the keyboard would be used is a user config option and the designer has no business touching it
• Anything listed under our docs that isn't strictly a user config option is made available to designers
• Specific additional options are made available to designers, such as USB vendor ID -- companies who have their own VID may want to override this one.

There are also some options that are listed in the docs, which I don't think should be listed in the docs as I don't understand why either designers or users should have any business touching them. For example:

• CONFIG_ZMK_USB_INIT_PRIORITY
• CONFIG_ZMK_BLE_INIT_PRIORITY

I have included such flags where I was uncertain as "designer configurable" out of caution.

While at it, I added a name to the EC11 trigger mode choice, and updated some values listed in the docs to be accurate.

Hopefully this more explicit approach is more "palatable" than #1886 and can get merged easier...

[Nick-Munnich]

Moved the USB PID/VID stuff back to the standard Kconfig file. I realised that the Kconfig.defaults files should only be used for stuff that both the user and the designer may want to configure. For designer-only and user-only, they have their respective .conf files.