fix: deprecate cloudflare platform.context in favor for platform.ctx

[vnphanquang]

Following #13851 I don't mind platform.context but it seems this is creating some inconsistency for people using create-cloudflare today. For the record, as far as I can remember, getPlatformProxy uses ctx, and in Cloudflare docs they use ctx as well.

Here are the places where platform.ctx typing is defined in create-cloudflare templates for Svelte:

• https://github.com/cloudflare/workers-sdk/blob/be766d267539b8f0a7ed6fc3dc979627f9eb7f04/packages/create-cloudflare/templates/svelte/workers/c3.ts#L79-L84
• https://github.com/cloudflare/workers-sdk/blob/be766d267539b8f0a7ed6fc3dc979627f9eb7f04/packages/create-cloudflare/templates/svelte/pages/c3.ts#L109-L114

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