Update CHANGELOG.md for v0.5.0 release

[RandomFractals]

• Add new v0.5.0 entry to changelog
• List new UX features added since the last published extension v4.2.0 (Use PRQL logo icon for .prql file extensions and display in file explorer and editor title bar #39 and above in issues list)
• Backfill v0.4.0 and 0.4.2 changes from releases