Flatten Text examples in RNTester to make them searchable (facebook#4…

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Summary:

# Changelog:
[Internal] - 

For some inexplicable reason, we had majority of `Text` test examples (42 out of 46) in RNTester stuffed into `<RNTesterBlock/>` components inside one huge "Basic" test case.

This was highly imbalanced, introduced extra nesting, cluttering the UI, but most importantly, none of those 42 out 46 test cases were searchable for.

This change flattens all of the corresponding nested test cases to the top level, making them into valid separate test cases, which are also searchable.

It also corresponds to the general structure we have in other test examples, such as `TextInput`.

Differential Revision: D57197676