Introduces TextVisitor to consolidate text traversal algorithms

[rozele]

The main motivation for this PR is to ensure each of the tree traversal algorithms we use inside RN Core Text supports 3rd party TextElements (e.g., Hyperlink). Right now, the logic is spread across a number of functions and each function would need to be checked individually to ensure it supports recursion through non-core TextElement view managers. Using visitor patterns, we can ensure that the base class appropriately handles recursion.

We have 3 different features that require a DFS traversal of the text tree:

• The textTransform algorithm needs to recurse the undefined paths of the Text sub-tree where the prop value changes to update.
• The backgroundColor algorithm needs to recurse the entire text tree to re-calculate highlighters any time the text changes or backgroundColor / foregroundColor props change within the tree.
• The WIP hit test algorithm needs to recurse the text tree to identify which span is pressed (not yet merged to PR since blocked on this one, see Issue5151 visitor rozele/react-native-windows#8)

Additionally, we have 4 "upward" traversal algorithms:

• When a RCTRawText changes it's text prop, we need to send an accessibility notification and update fast text.
• When a RCTRawText changes, we need to re-apply text transforms.
• When a background or foregroundColor changes, we need to notify each parent that it may require a highlighter for an optimization that skips calculating highlighters for text trees without foreground or background colors.
• In the WIP hit test algorithm, we need to notify parent nodes that a nested Text component is pressable for the optimization that selectively skips traversal of non-pressable sub-trees (not yet merged to PR since blocked on this one, see Issue5151 visitor rozele/react-native-windows#8)

Each of these algorithms follow the same pattern, so to reduce the potential for bugs, I'd like to consolidate these algorithms to use a visitor pattern.

While working on this diff, I discovered a bug in the textTransform logic where when we update the prop to null / undefined or add a child to a RCTVirtualText node with null / undefined, it should still attempt to inherit from ancestors.

Fixes #8460

Microsoft Reviewers: Open in CodeFlow

[chiaramooney]

Looks good to me, but will have to wait on another dev to look over the architecture decisions overall. One q, we filed an issue recently because textTransform style prop changes haven't been updating during Fast Refresh. With this overhaul change to Text, does that behavior now work?