@bem-react/classname: Unclear typing for element-bound class name formatters

[rrabmig]

Current behaviour:
The cn() helper can create a formatter in two different ways:

const cnBlock = cn('Block');
const cnBlockElement = cn('Block', 'Element');

Both values are currently typed as ClassNameFormatter, but the second form behaves differently and makes part of the callable API misleading.

/* block formatter*/
const cnBlock = cn('Block');

cnBlock('Element'); // => Block-Element

cnBlock('Element', { mod: true }); // => Block-Element Block-Element_mod

cnBlock({ mod: true }); // => Block Block_mod

/* element formatter*/
const cnBlockElement = cn('Block', 'Element');

cnBlockElement('Element2', { mod: true }); // => Block-Element Block-Element_mod

cnBlockElement({ mod: true }); // => Block-Element Block-Element_mod

Problem: the first argument 'Element2' in this call is accepted by types, but has no effect. This makes the type contract unclear. For an element-bound formatter, passing another element name is meaningless: the element is already fixed when the formatter is created and Block-Element-Element can't exist as well.

Suggestion: the two formatter creation patterns should have different return types:

ClassNameFormatter]= for block formatters created by the one-argument form
ClassNameElementFormatter- for element-bound formatters created by the two strings

cnBlockElement('Element2', { mod: true });
// should be a TypeScript error

Notes:

• Not a runtime bug
• Bump will be safe since first argument was doing nothing
• I am ready to fix that

[rrabmig]

#637 - my solution