Build for napi runtime when node-addon-api and napi_versions found

[lovell]

Once a native module has made the leap to using N-API via node-addon-api, attempting to build for node and electron ABI runtimes will fail.

This is, in part, due to a lack of a one-to-one mapping between node/electron ABIs and napi ABIs e.g. node ABI 57 maps to napi ABI 1, 2, 3 and 4.

This change is fully backwards compatible. Existing modules that do not both depend on node-addon-api and specify napi_versions carry on as before.

[lovell]

I've updated this PR so the napi runtime is additive. The internals have been corrected to use target rather than abi to ensure we're passing the expected value to prebuild's -t flag, which then allows node-addon-api to build correctly.

I tested this with release candidate v3.0.0-rc1 of the farmhash module and it generated the expected prebuilds when using Node 10 and with napi_versions = [2, 3, 4] (napi 4 was correctly ignored as this requires Node 12):

farmhash-v3.0.0-rc1-electron-v64-linux-x64.tar.gz
farmhash-v3.0.0-rc1-electron-v69-linux-x64.tar.gz
farmhash-v3.0.0-rc1-electron-v70-linux-x64.tar.gz
farmhash-v3.0.0-rc1-napi-v2-linux-x64.tar.gz
farmhash-v3.0.0-rc1-napi-v3-linux-x64.tar.gz
farmhash-v3.0.0-rc1-node-v64-linux-x64.tar.gz

[vweevers]

@lovell Why is it a requirement to have node-addon-api in your dependencies? What happens when you don't use that C++ API?

[lovell]

It's optional but highly recommended; happy to remove that check if napi_versions is considered a strong enough indicator.

I think of node-addon-api as the N-API equivalent of nan, without which maintaining native modules would be a(n even greater) burden.

[vweevers]

There are quite a few modules that don't use node-addon-api, but napi-macros (many of these also don't use prebuild[-ci], but that's besides my point that node-addon-api is not the only solution).

[lovell]

napi-macros looks useful, I hadn't seen that before, thanks for the tip. I've updated this PR to remove the dependency check and rely on the presence of napi_versions only for now.

[vweevers]

Before this PR, we didn't support N-API at all, correct? Meaning we can safely release this as semver-minor.

[lovell]

That's correct, this change can be part of a minor increment.

[vweevers]

Two nitpicks, LGTM otherwise.

[vweevers]

3.1.0