Typings: add generic support to parse function

[ericmorand]

Currently, parse is declared as returning DotenvParseOutput. This forces people wanting to have typed environmental configuration to typehint the result of parse as any:

type Configuration = {
    FOO: string
};

const configuration: Configuration = parse(readFileSync('.env')) as any;

This looks not very elegant and could be solved easily by having parse function being generic:

type Configuration = {
    FOO: string
};

const configuration= parse<Configuration>(readFileSync('.env'));

[motdotla]

Thanks again for this @ericmorand. Going out as part of 10.0.0