Add ExPlat logic around customize-store feature

[ilyasfoo]

Changes proposed in this Pull Request:

This PR:

• Adds ExPlat logic and code to enable customize-store feature
• Improves task removal code via JS since appearance plugin was added before our removal filter is added
• Adds isCustomizeStoreEnabled global variable to access CYS flag via JS

How to test the changes in this Pull Request:

Verify control experience

1. If testing in old site, make sure customize-store feature flag is disabled
2. Go to Homescreen
3. Observe Choose your theme task in tasklist
4. Ensure there's no duplicate Choose your theme task
5. Click on the task
6. Ensure you're redirected to WPCOM Themes page.

Verify treatment

1. Open up CLI for your site, i.e: https://yoursite.wpcomstaging.com/_cli
2. Run option set woocommerce_admin_install_timestamp 1703116800
3. Run transient set abtest_variation_woocommerce_wooexpress_cys_launch_v1 treatment
4. Go to Homescreen
5. Observe Customize your store task in tasklist

Other information:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?

FOR PR REVIEWER ONLY:

• [ ] I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

[github-actions[bot]]

Size Change: +38 B (0%)

Total Size: 196 kB

Filename	Size	Change
./build/index.js	122 kB	+38 B (0%)

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./build/53.js` | 1.05 kB | | `./build/index.css` | 883 B | | `./build/marketing.js` | 58 kB | | `./build/payment-gateway-suggestions.css` | 1.25 kB | | `./build/payment-gateway-suggestions.js` | 6.45 kB | | `./build/plugins.js` | 3.92 kB | | `./build/style-index.css` | 2 kB | | `./build/style-marketing.css` | 805 B |

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[ilyasfoo]

Thanks for the review, @psealock! However, we've decided to forgo ExPlat p1702940957786639-slack-C053716F2H2.

I've created a follow-up PR instead: https://github.com/Automattic/wc-calypso-bridge/pull/1399. Closing this PR