text/template: document function name rules and refactor goodName

[kpym]

This change enhances the documentation by specifying permissible function names in the Funcs function of both text/template and html/template.

Additionally, the goodName function has been refactored to improve readability and maintainability, ensuring it allows only letters, underscores, or digits, with the constraint that a digit cannot be the initial character.

While the primary modifications are in text/template, they also impact html/template, as it utilizes goodName internally.

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