Url search params and remove multiple

[GrosSacASac]

remove multiples
remove unused qs and querystring dependency

files and fields are always arrays to avoid type errors
use modern URLSearchParams https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams internally

[GrosSacASac]

fixes #634, #485, #400, #292

[tunnckoCore]

I will review it soon.

[GrosSacASac]

An utility to force get singular values

const firstValuesOfFields = fields => {
    return Object.fromEntries(Object.entries(fields).map(([key, value]) => {
        return [key, value[0]];    
    }));
};

// ...

form.parse(request, async (err, fieldsM /*files*/) => {
        const fields = firstValuesOfFields(fieldsM);