reorg: extract pptx.text subpackage

No code changes, strictly migrating pptx/text.py into its own
sub-package in preparation for further additions and extractions.
rfctr: refactor DescribeTextFrame

Modernize fixtures mostly.
txt: add TextFrame.vertical_anchor getter
docs: add Legend.font analysis
acpt: add scenario for Legend.font
reorg: extract chart.Legend into module
cht: add Legend.font
docs: document shrink-to-fit analysis
acpt: add scenario for TextFrame.fit_text()

* modernize phrasing of TextFrame.word_wrap scenario
txt: add TextFrame.fit_text()
txt: add TextFrame._best_fit_font_size()
txt: add TextFrame._extents
txt: add TextFitter.best_fit_font_size()
txt: add TextFitter._best_fit_font_size()
txt: add TextFitter._fits_inside_predicate()
txt: add _BinarySearchTree.from_ordered_sequence()
txt: add _BinarySearchTree.find_max()
txt: add TextFitter._wrap_lines()
txt: add TextFitter._break_line()
txt: add TextFitter.__iter__()