Expose low-level API to use tailwind-merge logic for custom use cases

[dcastil]

There are some uses cases that are too custom for tailwind-merge to handle but that would benefit from leveraging tailwind-merge's understanding of Tailwind CSS classes and conflicts between them.

Some example use cases:

• How to get the conflicting classes? #384
• expose a parseClassList function derived from the existing mergeClassList function #373
• Add config option to ignore variants/modifiers #440

tailwind-merge could expose a low-level API that makes it possible to use its internal logic for those use cases.

Some important aspects this feature must fulfill:

• Should not increase bundle size for the vast majority of tailwind-merge users that won't use this code
• Should be as powerful as possible to enable as many use cases as possible
• Should not increase package maintenance complexity too much

[dcastil]

So far I'm thinking of a new exported function like createTailwindMergeUtils or extendTailwindMergeUtils or so that exports a bunch of util functions to get all the information that tailwind-merge uses internally.

[dcastil]

One idea is to allow to replace the splitModifiers function call in https://github.com/dcastil/tailwind-merge/blob/v2.3.0/src/lib/merge-classlist.ts#L23-L28 (I'd rename it to something like splitClassName or getClassNameParts). That could unlock some powerful low-level functionality.