fund: support multiple funding sources

[ljharb]

What / Why

Adds support for multiple funding sources in package.json.

References

See npm/rfcs#68

[ruyadorno]

It would be nice to add more tests with a variety of single vs multiple funding sources tests:

• In test/tap/utils.funding.js it would be nice to stress it a bit more with cases such as duplicated urls within the same array, also varying between shorthand string vs object (with and without type), etc
• In test/tap/fund.js it would be nice to add a fixture using multiple funding items.

[ljharb]

@ruyadorno updated! i also added tests for the "open URL" multiple sources case, and fixed a bug.

(There's a possible issue with command arg parsing - npm fund . --browser causes the args to be ['.'] and opts.browser to be 'true', but npm fund --browser . causes args to be [] and opts.browser to be . - iow, it doesn't seem to be able to detect that an option is a boolean arg, when it's followed by other args. That seems very out of scope of this PR to fix, though)

[ruyadorno]

just one very small doc request but that can even be added by the release manager 👍 LGTM

[ruyadorno]

That seems very out of scope of this PR to fix, though

agreed 👍 + the team has been very busy reworking the entire handling of configs, so the implementation might change a lot in npm@7 - not to say it will fix itself but just that it's not the best moment to play with that 😬