Support builds that completely exclude I2CDev support

[fhunleth]

This removes both the Elixir module and the NIF. There's no longer any
unintended NIF builds or loads or anything extra.

To enable this, either set the :default_backend application
environment key or force it by setting :include_i2c_dev. Most users
probably want:

config :circuits_i2c, default_backend: MyI2CBackend

This fixes the issue reported at https://elixirforum.com/t/circuits-sim-in-a-release/72076/3.

[Copilot]

Pull Request Overview

This PR restructures the Circuits.I2C project to make the NIF-based I2C backend optional, allowing builds without the native Linux dependencies. The changes move NIF-related code to a separate i2c_dev directory and introduce configuration options to control whether the NIF is built.

• Moved all NIF-related C code and Elixir modules to a new i2c_dev subdirectory
• Added application configuration to conditionally include or exclude the I2CDev backend
• Enhanced the NilBackend to provide a complete implementation when I2CDev is disabled

Reviewed Changes

Copilot reviewed 7 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
mix.exs	Restructured project configuration to conditionally include NIF compilation based on build_i2c_dev?() function
lib/i2c/nil_backend.ex	Added Bus protocol implementation with error-returning stubs for all I2C operations
i2c_dev/c_src/i2c_nif.c	Fixed missing closing brace and added errno setting for test backend
REUSE.toml	Updated file path references to reflect new i2c_dev directory structure
README.md	Added documentation for new application configuration options
Makefile	Updated all C source paths and removed "disabled" mode, now only supports "normal" and "test"
.formatter.exs	Added i2c_dev directory to formatter input paths

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