feat!: Add extends syntax

[phated]

@sttk this is my (very hacky) initial work on my vision for extends syntax. Please review and make any suggestions. I still need to find answers on all the TODOs and write a ton of tests (which will likely result in refactoring).

Closes #101 (supersedes it)

[sttk]

@phated This pr is generally good! I'm sorry that I'm slow on the uptake. Wrong idea (and my poor english) had hindered my understanding. As you commented, findUp option of fined can solve the issue of loading a config file in cwd found up.

BTW, there are one thing I'd like you to confirm. Will the way to specify .configFiles of constructor be changed to use array? (I think it's better for those order.)

[phated]

@sttk don't worry about it! Your work heavily influenced this code.

BTW, there are one thing I'd like you to confirm. Will the way to specify .configFiles of constructor be changed to use array? (I think it's better for those order.)

I think it's important to switch the configFiles values to be array because we are now loading and merging the configs. My current implementation will look at each entry until one is found, then it will load that file and switch to merging using extends property.

[phated]

@sttk this has taken me a long time to come back to, but I am hoping you can review this PR because it adds the extends syntax that we plan to support in gulp-cli. Thanks 🙏

[phated]

Oh, and this is a breaking change because we changed the syntax to an array of fined objects (instead of a single object).

[sttk]

@phated OK, I'll review this PR this weekend.

[sttk]

@phated I've reviewd this PR, and I didn't find any problem. Great!