Allow for Caddyfile to implicitly set prefer value for encode directive

[polarathene]

Creating an issue for a recent discussion on encode directive docs PR for upcoming 2.4 release.

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Proposal

For a Caddyfile, encode.prefer could be set implicitly.

- encode zstd gzip {
-  prefer zstd gzip
- }
+ encode zstd gzip

That would be intuitive to the user and consistent with the new file_server.precompressed feature.

Justification

Without prefer, only the first supported client encoding is negotiated (often gzip).

Impact of implicit preference:

• Backwards compatibility concern already applies to encode adopting sane defaults.
• Consistency:
  • The new file_server.precompressed has implicit preference.
  • Minimizing config verbosity and good defaults are goals of Caddy.
• Caddy vs Clients:
  • Caddy: Only zstd and gzip are officially supported (50% chance gzip remains preferred).
  • Browser: Not a single browser supports zstd officially.
  • Other clients: If gzip is supported it often took priority, even if unintentional (eg cURL).

[francislavoie]

/cc @ueffel