Exclude dev files from production builds at the package level (#47365)

Add production-exclude rules for source files (.ts, .tsx, .jsx, .scss)
to package .gitattributes files. This ensures dev files are excluded
when packages are installed into plugins via Composer.

Also updates the package skeleton template with commented examples of
common exclusion patterns.

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Co-authored-by: Brad Jorsch <brad.jorsch@automattic.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/26050119273

Upstream-Ref: Automattic/jetpack@10d726e

Author: anomiex

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 This is an alpha version! The changes listed here are not final.
 
 ### Changed
+- Exclude development files from production builds.
 - Update package dependencies.
 
 ## [7.21.1] - 2026-05-11