Accessibility: Add the word `Theme:` to img alts for themes on settings modal

[sejas]

• Closes https://github.com/WordPress/wordpress-playground/issues/619

Description

It adds modifies the theme image alts on settings modal so they are different from the text below.

Testing instructions

• In a terminal run:
  • yarn install
  • composer install
  • yarn wp-env start # to start the main server
• In a new terminal run:
  • cd source/wp-content/themes/wporg-wasm/src/wasm-demo
  • nvm use v14.21.3
  • yarn install
  • yarn start # to start the wasm demo block
• Go to /wp-admin
• Activate the Wasm theme if it's not active yet.
• Create a new page
• Embed the block wasm demo
• View the forntend page
• Click on settings button
• Inspect the theme images
• Observe the alt images are different from the text below.
• Couldn't reproduce the issue using axe chrome extension

Screenshot

Before

After