Add composer build command to package plugin zip

[adamsilverstein]

Fixes #21

What

Adds a single command to build a distributable zip for the WordPress.org plugin repository:

composer build

This produces dist/client-side-media-experiments.zip.

How

• bin/build-zip.sh: copies runtime files via rsync --exclude-from=.distignore into a single top-level client-side-media-experiments/ directory (as required by WordPress.org) and zips it into dist/.
• composer.json: adds a build script pointing at the shell script.
• .distignore: extended to also exclude .claude, .wordpress-org, bin, dist, and README.md (the WordPress.org repo uses readme.txt). Since the release deploy workflow (10up/action-wordpress-plugin-deploy) uses the same .distignore, local builds and SVN deploys now share one exclusion list.
• .gitignore: ignores dist/.
• README.md: documents the command under a new Development section.

Verification

Ran composer build twice (idempotent). Zip contents:

client-side-media-experiments/
client-side-media-experiments/client-side-media-experiments.php
client-side-media-experiments/js/cross-origin-isolation-coep.js
client-side-media-experiments/js/heic-support.js
client-side-media-experiments/includes/heic-support.php
client-side-media-experiments/includes/settings.php
client-side-media-experiments/includes/cross-origin-isolation.php
client-side-media-experiments/readme.txt

No development files included; single top-level plugin directory.

[coderabbitai]

Caution

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch add/build-zip-command

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