Adding `ard_emmeans_mean_difference()` #600

Job	Run time
Version Bump 🤜🤛	0s
Audit Dependencies 🕵️‍♂️ / oysteR scan 🦪	1m 41s
Coverage 📔 / Coverage 📔	2m 41s
Coverage 📔 / Covtracer 🐄	0s
gitleaks 💧 / gitleaks 💧	4s
gitleaks 💧 / PII Check 💳	0s
R CMD Check 🧬 / ghcr.io/insightsengineering/rstudio, version latest	2m 40s
Roxygen 🅾 / Manual pages check 🏁	2m 34s
Check URLs 🌐 / Validate Links 🔎	7s
Grammar Check 🔤 / Check 📝	12s
Spell Check 🆎 / Check spelling 🔠	1m 37s
License Check 🃏 / Check 🚩	34s
Version Check 🏁 / Version check 🏁	3s
Style Check 👗 / Check code style 🔠	1m 41s
Version Check 🏁 / Emoji in NEWS.md 📰	6s
Coverage 📔 / Publish coverage report 📰	1s
R CMD Check 🧬 / Upload build tar.gz	1s
R CMD Check 🧬 / Publish JUnit HTML report 📰	1s
	14m 3s

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