Nested prompt visualization

[gavento]

Add FormatStr class that acts as a string with format fields (like in Python f-string), but remembers the replaced parts and distinguishes them (colors them differently) in the context log.

TODOs:

• [x] Play with the colors, likely color the background (with transparency) rather than coloring the text (except for unfilled fields)
  • [x] Make it also work for nested fmts (apply color of different transparency?) and on dark backgrounds.
• [x] Improvements: joining and addition of FormatStrs?
• [x] Add a better demo to vis notebook, also one using x.format(**locals())
• [x] Rename to FormatStr for pythonic consistency?
• X (not needed) Add some styling to browser CSS (rather than inline styles?)
• X (low priority) Add a separate type for formatting (rather than HTML?)

An example:

[gavento]

Would merge as is. Not sure if this should become part of core (interlab.text) or contrib/experimental (but could be used in core agents and memories - should be non-intrusive if used at the highest level of formatting).