Rename `JsonParser.getText()` as `.getString()` [JSTEP-6]

[cowtowncoder]

As per JSTEP-6, we will rename references of "Text" in accessors for (JSON) String values to use "String" instead.
In some cases old method will be left deprecated but available (getText() f.ex), whereas in others method is just renamed (new one added, old removed).

Specifically:

• getText(): deprecate, add getString() as replacement
• getText(Writer): rename as getString(Writer) (no deprecated method left)
• getTextCharacters(), getTextLength(), getTextOffset(): deprecated, add new variants with name getStringXxx()
• hasTextCharacters(): rename as hasStringCharacters() (no deprecated method left)
• nextTextValue(): rename as nextStringValue() (no deprecated method left)