Helper function for building text and track offset

One common task for almost all readers is to keep track of the text content and offset. This is a tedious task and error-prone. So it would be helpful if Forte can help this. This can be designed to work well with the text_replacement_func.

The issue does not name a file or test. Start by inspecting the reader implementations and the text_replacement_func references to understand how text content and offsets are currently maintained; done should include an agreed helper design that works with text replacement and is covered by tests.

Written by the indexing model from the issue text.