Implement {rix} environment configuration for reproducible R environments

[Copilot]

This PR adds reproducible environment management using the rix package, which generates Nix expressions for consistent R environments across different systems.

Changes Made

New Files

• rix.R: Main configuration file that loads the rix package and generates two environments:
  • Runtime environment (default.nix): Minimal setup for using ojoslackr with core dependencies
  • Development environment (shell.nix): Complete development setup with RStudio and all dev tools

Updated Files

• .Rbuildignore: Excludes rix.R and generated Nix files from package builds
• .gitignore: Excludes generated default.nix and shell.nix from version control
• README.Rmd: Added comprehensive documentation on using the rix environments

Environment Details

Runtime Environment

Includes essential packages for using ojoslackr:

• Core dependencies: ggplot2, grid, slackr
• Common usage packages: here, tidyverse
• GitHub packages: ojodb, ojoslackr

Development Environment

Includes everything from runtime plus development tools:

• Package development: devtools, roxygen2, pkgdown, testthat
• Code quality: styler, lintr, covr
• Documentation: knitr, rmarkdown
• IDE integration: RStudio

Usage

# Generate Nix environment files
source("rix.R")

# Use environments
# nix-shell default.nix    # Runtime
# nix-shell shell.nix      # Development

This ensures all contributors and users have identical R versions and package dependencies, eliminating "works on my machine" issues.

Fixes #1.

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