API: Add `Styler.to_html`, for saving output to HTML file
- [x] closes #11700 and closes #13379

## `Styler.to_html` method

This PR adds the above method to `Styler`. 

This is a **minimal** introduction with limited arguments to provide just the core file io functionality and basic options, either to write a fully structured HTML file or just the `Styler.render` output. It is very easier to extend in follow ons.

#### Jinja 2 template inheritance

Due to the more complex logic, the jinja2 templates were split into smaller components:

 - html_style.tpl: this just deals with the `<style>` html element
 - html_table.tpl: this just deals with the core `<table>` element including (or excluding) style identifiers
 - html.tpl: this is the controlling template which imports the above based on what is required.

It is more performant to have a separate temple that excludes styling elements by about 33%, rather than include the logic in a single template. I think it is easier to maintain as well.

#### Tests

A new `test_to_html.py` file is added. Some tests are moved from `test_style.py` if they more relate to HTML generation. Some new tests are added.

#### Objective

Ultimately the medium-term aim is to deprecate `DataFrame.to_html`. `Styler` is currently faster at rendering HTML than `DataFrame.to_html` (#39972), and this is further improved in this PR.
There is a standalone purpose for wanting to create HTML to file from Styler.
This PR complements `Styler.to_latex` whose aim is also to deprecate `DataFrame.to_latex`.

@jreback ping: greenish, failure unrelated thanks @attack68 