TEXT-138 TextStringBuilder append sub-sequence not consistant with Appendable
1 considered the end index as exclusive because its exclusive as per the definition done in Appendable.
2 considered start index not to be same as the end index because start index is inclusive, end index is not.
3 there is an overloaded append function in TextStringBuilder which takes String, int, int as parameters. The last parameter here is length. I didn't touch this because, its NOT an overridden function from Appendable.
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