CI: add greeting workflow for first-time contributors

[RishavTiwari25]

This Pull Request fixes/closes #5126 .

It changes the following:

• Adds .github/workflows/greetings.yml.
• Automates welcome messages for first-time issue reporters with helpful documentation links.
• Automates welcome messages for first-time PR authors with contribution guidelines and expectations.
• Utilizes the official actions/first-interaction@v1 action with safe, explicitly defined write permissions.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,073	0
Ignored	2,072	2,072	0
Failed	818	818	0
Panics	0	0	0
Conformance	94.54%	94.54%	0.00%

Tested main commit: 70409a5052984325dccfdc5f6520818568a81f39
Tested PR commit: eb9e49fe39261a5216bd943c544da308d05fba16
Compare commits: 70409a5...eb9e49f

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.31%. Comparing base (6ddc2b4) to head (eb9e49f).
⚠️ Report is 886 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5127       +/-   ##
===========================================
+ Coverage   47.24%   59.31%   +12.07%     
===========================================
  Files         476      563       +87     
  Lines       46892    62795    +15903     
===========================================
+ Hits        22154    37249    +15095     
- Misses      24738    25546      +808     

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