rewrite desk_height_sensor.h as external component

[phntxx]

Hi 👋🏻

Given that custom components for esphome are deprecated, I decided to rewrite desk_height_sensor.h as an external component instead.

This works by essentially adding some boilerplate code around the preexisting codebase of desk_height_sensor.h and has the exact same functionality as if one were to include desk_height_sensor.h within their esphome yaml file.

[iMicknl]

@phntxx thanks, I will have a look! Just be aware there is a v2 branch (which we should merge soon), where we have already additional improvements. #67 :)