feat: BUTTON_BUILTIN alias for USER_BTN

[zfields]

Create an alias for those new to STM32duino environment.

USER_BTN is commonplace in STM32duino, however there are several instances of BUTTON_BUILTIN across other Arduino BSPs. The naming convention of BUTTON_BUILTIN is more closely aligned to LED_BUILTIN, and there appears to be some precedent.

Examples:

• https://github.com/espressif/arduino-esp32/blob/master/variants/micro_s2/pins_arduino.h#L75
• https://github.com/esp8266/Arduino/blob/master/variants/espectro/pins_arduino.h#L40
• https://github.com/f32c/arduino/blob/master/hardware/fpga/f32c/variants/FleaFPGA-Uno/variant.h#L17
  ... as well as several other examples that can be discovered by Googling, "BUTTON_BUILTIN" arduino.

Since ST is the clear leader in boards having a built-in button, it has a unique opportunity to help reinforce the semi-standardized precedent BUTTON_BUILTIN by providing an alias for all boards that support a USER_BTN.

[fpistm]

Hi @zfields
6 years I'm working in Arduino Ecosystem and honestly, I never seen this definition.
Searching with google give me only 2 pages. It seems mainly defined by ESP.
All standard Arduino examples use the USER_BTN definition.

For Ardunio, no reference:
https://github.com/search?q=org%3Aarduino+%22BUTTON_BUILTIN%22
https://github.com/search?q=org%3Aarduino-libraries+BUTTON_BUILTIN

Same for Adafruit:
https://github.com/search?q=org%3Aadafruit+BUTTON_BUILTIN

And on all GitHub, only 327 occurrences in code:
https://github.com/search?p=32&q=%22BUTTON_BUILTIN%22&type=Code

I think it is really not a good idea to add it, the Arduino standard is USER_BTN. Using BUTTON_BUILTIN in a code would break Arduino compatibility. If you really want have it as a standard it should be submit first to Arduino then it will benefit to all other project following Arduino standard.

[ABOSTM]

I am in line with @fpistm comments.
I prefer to reinforce the Arduino standard USER_BTN (alone),
instead of having a second semi standard definition for the same thing, introducing confusion.
Maybe, library using BUTTON_BUILTIN should stick to Arduino standard (USER_BTN ), or create the alias on theyr own ...