feat: Expose isMergeKey and MERGE_KEY constant

[danielbayley]

Trying to use isMergeKey for something… Is there a reason it isn’t exposed alongside isScalar etc? Also, seems like MERGE_KEY should be a shared symbol, no?

[eemeli]

Probably best to roll back this change.

Currently, main has changes that will only get released as a v3 release of the library, so if you want/need isMergeKey to be available on v2, this PR needs to not include breaking changes.

For v3, I intend to be much stricter on the values that can be put in a document, and so for something like merge keys, end up with an instanceof check.

[danielbayley]

for something like merge keys, end up with an instanceof check.

I thought about instanceof, but that operator doesn’t work on Symbols, which seem like the best fit for merge keys, surely?

this PR needs to not include breaking changes.

Would exporting MERGE_KEY as a shared Symbol even be a breaking change, given that it seems to only currently exist within this file?

Probably best to roll back this change.

Have rolled back to just exporting MERGE_KEY as string for now/v2…

[eemeli]

As I mention in my previous review, I intend to make type checking stricter in v3. To that end, we shouldn't be exposing MERGE_KEY, but providing and enforcing the use of a new Merge class for this purpose.

Suggested change

	export const MERGE_KEY = '<<'
	const MERGE_KEY = '<<'

[eemeli]

This isn't really making the code more legible.

Suggested change

	test: new RegExp(`^${MERGE_KEY}$`),
	test: /^<<$/,