feat: Migrates the boundaries-click sample.

[willum070]

This PR makes the following changes:

• Migrates boundaries-click to js-api-samples.
• Updates to use the gmp-map element.
• Rewrites HTML generation code to be XSS-safe.

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• samples/boundaries-click/index.html:7, tag maps_boundaries_click_event
• samples/boundaries-click/index.ts:7, tag maps_boundaries_click_event
• samples/boundaries-click/index.ts:14, tag maps_boundaries_click_event_handler
• samples/boundaries-click/index.ts:47, tag maps_boundaries_click_event_add_layer
• samples/boundaries-click/index.ts:105, tag maps_boundaries_click_event_style
• samples/boundaries-click/style.css:6, tag maps_boundaries_click_event

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