fix(superagent-wrapper): support non-/ api base paths

[bitgopatmcl]

This should fix #337. I initially tried to use an alternate URL constructor like:

const url = new URL(substitutedPath, basePath);

and was surprised to learn that this actually has the exact same behavior of stripping the base path. It seems like it should be possible to make the substituted path a relative one, but that has issues as well.

Ultimately I pulled in posix.join from path. This works, but may potentially introduce issues with some web bundlers if they don't provide an implementation of Node's path module.

[ericcrosson-bitgo]

Preserves the base url after path-param substitution 🚀

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🎉 This PR is included in version @api-ts/express-wrapper@1.0.12 🎉

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