feat: Add extend property

[Richienb]

Fixes #493

[Gerrit0]

Is this intended to be in draft status?

[Richienb]

@Gerrit0 Probably not. I made it and forgot it. I'll finish up the PR soon.

[Richienb]

@Gerrit0 How should we detect circularity?

[Gerrit0]

The simplest way is probably to just keep a Set<string> of the resolved paths that have been loaded, and then error if we see the same path twice. This would technically have some false positives (e.g. extends: ['./a.json', './a.json'], but that's likely a mistake in the first place.

[maneetgoyal]

Hi @Gerrit0, is this PR awaiting any further corrections?

[Richienb]

@maneetgoyal The last thing that needs to be implemented is circularity detection.

[maneetgoyal]

Hi @Richienb, thanks for picking up this PR. It seems very useful. Any time estimate on this will be of great help.

[Richienb]

@maneetgoyal I've just finished the last part of the PR. After a bit of testing, it will be ready.

[Gerrit0]

Logic looks good to me 👍 I should be able to pull this in this weekend.

[Gerrit0]

Ah, of course this was built before the options refactor - I'll pull the changes in to the refactored code.