Allow prioritized list in tag directive

[klauspost]

Tags are tried in order and we maintain the existing fallback.

Adds tags as alias.

[Copilot]

Pull request overview

This PR extends msgp’s tag-directive handling to support a prioritized list of struct tags (tried in order) while preserving the existing fallback behavior to msg and then msgpack. It also adds tags as an alias directive and includes a generated-fixture test to validate the selection rules.

Changes:

• Change field-tag configuration from a single tag name to a priority-ordered list (tagNames).
• Update the tag directive to accept multiple tags (including comma-separated lists) and register tags as an alias.
• Add _generated fixtures + tests to validate priority selection and fallback behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
parse/getast.go	Reads field names from the first matching non-empty tag in a configured priority list, then falls back to msg and msgpack.
parse/directives.go	Adds tags directive alias and parses tag/tags arguments into a priority list stored on FileSet.
_generated/custom_tags.go	Adds a go:generate fixture demonstrating tags primary,fallback usage and fallback behavior.
_generated/custom_tags_test.go	Tests encoding/decoding and verifies wire keys reflect the configured tag priority and fallback rules.

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