Proposal: Optimize func goodName(name string) bool in text/template

[kpym]

text/template: clarify function name rules and improve readability of goodName

This change adds documentation to clarify the rules for permissible function names.
It also refactors the goodName function to improve readability and maintainability, ensuring it adheres to the logic of allowing only characters, underscores, or digits, but not starting with a digit.

While the primary changes are in text/template, they also affect the html/template package, as it uses goodName internally.

Should I append a benchmark to demonstrate the observed performance improvements? While the speed gain is a bonus rather than the primary motivation, I can provide one if necessary.

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