support parsing uint64

[xwjdsh]

Issue: add link to pelletier/go-toml issue here
spf13/viper#1221

Explanation of what this pull request does.

For the following code snippet, an error will be returned, (2, 5): strconv.ParseInt: parsing "18446744073709551615": value out of range, this PR deals with cases where the value is greater than math.MaxInt64 and less than or equal to math.MaxUint64.

package main

import (
	"fmt"

	"github.com/pelletier/go-toml"
)

type Config struct {
	V uint64
}

func main() {
	doc := []byte(`
V = 18446744073709551615
`)

[pelletier]

Thank you for the patch!

Not sure what version of go-toml Viper uses, but if possible I recommend using go-toml/v2, where that bug was not present.