feat: Migrates the polyline-simple sample.

[willum070]

This PR does the following things:

• Migrates the polyline-simple sample to js-api-samples.
• Updates to use dynamic loading and the gmp-map element.
• Makes no other changes, as this sample is perfect in its simplicity.

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• samples/polyline-simple/index.html:7, tag maps_polyline_simple
• samples/polyline-simple/index.ts:7, tag maps_polyline_simple
• samples/polyline-simple/style.css:7, tag maps_polyline_simple

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