feat(decoder): support multiple tag names in order

[DarkiT]

Add support for comma-separated tag names in DecoderConfig.TagName. The decoder will try each tag name in order and use the first valid tag value found.

Example:
config := &DecoderConfig{
TagName: "mapstructure,yaml,json",
Result: &Config{},
}

This allows more flexible struct field mapping by supporting multiple tag formats while maintaining backwards compatibility with single tag name usage.

The decoder will:

1. Try each tag name in the specified order
2. Use the first valid tag value found
3. Fall back to field name if no tag found
4. Skip field if IgnoreUntaggedFields is true and no tag found

Changes:

• Update DecoderConfig.TagName comment to document multiple tag support
• Modify decodeStructFromMap to handle comma-separated tag names
• Keep backwards compatibility with single tag name usage

[derekperkins]

@sagikazarmark any chance you could take a look at this?

[sagikazarmark]

I've put this on my list, I'll try to get this reviewed shortly.