[R-package] factor out lgb.check.r6.class()

[jameslamb]

The R package has many places where it needs to check that an object is a special LightGBM R6 class (lgb.Booster, lgb.Dataset, or lgb.Predictor). Today, it is inconsistent in the way that it does this. Some code uses special functions like lgb.is.Booster() which call a one-line utility function lgb.check.r6.class(). Other code calls lgb.check.r6.class().

This code proposes factoring out lgb.check.r6.class() completely. I think this has the following benefits:

• removes a layer of indirection / opportunity for inconsistency
• makes the code easier to read and understand

# before
if (!lgb.check.r6.class(object = train_set, name = "lgb.Dataset"))

# after
if (!lgb.is.Dataset(train_set))

Since what lgb.check.r6.class() does is so simple, I think that the added indirection it introduces is not worth it, and that the R package's code will be a bit simpler as a result of this change.

[StrikerRUS]

LGTM!

[github-actions]

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