feat: Refactor request to make sure we only use available transports

[benjackwhite]

Changes

Possibly fixes #1079

• Generate the list of available request transports
• Try to use the one requested, falling back to whatever is available

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

[vercel]

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	May 7, 2024 4:46pm

[github-actions]

Size Change: +1.4 kB (0%)

Total Size: 974 kB

Filename	Size	Change
dist/array.full.js	234 kB	+348 B (0%)
dist/array.js	131 kB	+349 B (0%)
dist/es.js	132 kB	+349 B (0%)
dist/module.js	132 kB	+349 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	12.2 kB
dist/recorder-v2.js	108 kB
dist/recorder.js	108 kB
dist/surveys-module-previews.js	57.2 kB
dist/surveys.js	58.8 kB

_{compressed-size-action}

[pauldambra]

cheekily included a fix to save myself another PR

tested this locally with api_transport explicitly set to each of the three options it takes and could capture events and see the transport flick between xhr and fetch so 👍