Refactor internal/gpx and internal/xlsx using generics

[dolmen]

Packages internal/gpx and internal/xlsx contain duplicated code that could be unfied using generics.

[dolmen]

I just did an experiment using this util function that enables to use range-over-func (requires to compile with GOEXPERIMENT=rangefunc on Go 1.22):

func iterTokens[Token any, Tokenizer interface{ Token() (Token, error) }](tok Tokenizer) iter.Seq2[Token, error] {
	return func(yield func(Token, error) bool) {
		for {
			token, err := tok.Token()
			if err == io.EOF {
				break
			}
			if !yield(token, err) {
				break
			}
		}
	}
}

Result of the experiment: I don't think its worth it for now (especially because of GOEXPERIMENT).

[dolmen]

See also golang/go#68259 which I just filled and which proposes to add a Tokens() iter.Seq2[Token, error] on encoding/xml.Decoder.

[dolmen]

I expect this will rejected according to @muktihari's comment on another PR:
#17 (comment)

So, closing.