Update default n-gram length for Text Transform to match default text recipe

[daholste]

@justinormont and the text team tuned default n-gram lengths for the default text recipe in the internal repo

These defaults are:
Word -- bigrams (w/ unigrams)
Character -- trigrams (w/o unigrams and bigrams)

One chart from his findings:

The line w/ the light blue call-out represents current ML.NET defaults (Unigram + Trichar)
The line w/ the light green call-out is the requested change (Bigram + Trichar)
The line w/ the pink call-out shows the Trigram+Trichar is better in terms of accuracy, but with a time hit, and accuracy has a cross over at NumIterations > 8 for Averaged Perceptron learner.

[rogancarr]

Related to #2802

[zeahmed]

@justinormont and @shauheen, do you want this to go in V1.0?

[justinormont]

That's up to @shauheen. I'd say yes, as there's strong upsides of accuracy. You'll notice the large jump in accuracy (y-axis) when we move from the blue to green lines in the above graph.

The power of defaults should never be underestimated.

Related: #2305

[najeeb-kazmi]

Tracking in #4749