[ENH] Define Triangle Styler #1331

Another multi-part issue...

Referencing #1324, it's time to define a Styler for the Triangle. A Styler is a class that enables aesthetic configuration of the Triangle for output to various displays, in HTML, LaTeX, etc. For the Pandas styler, refer to the following tutorial:

https://pandas.pydata.org/docs/user_guide/style.html

This can be accessed via Triangle.to_frame().style.format(...), but challenges remain:

• The Pandas styler cannot access the actuarial elements of the Triangle, such as latest diagonal, lower triangle (see [ENH] Different background for upper and lower triangles #1324), which puts a high burden on the user to define complex, custom functions to format these elements.
• No support for origin/development indexes

Concatenation

Another cool feature that we can adapt is the concatenation of stylers. For example, applying this to a Triangle(s) with a compatible table of LDFs should give us the ability to build the exhibits in Friedland, such as:

Builtin styles

The Pandas styler has builtin styles. The Triangle styler can have its own builtins, where we can format things like latest diagonal, lower triangle, upper triangle, origin, development, etc. This can be done via:

Triangle.style.highlight_lower_triangle(color="blue")

Which can be the result of solving #1324.