feat: move away from Kindlegen to Kindling

[BoboTiG]

All <br> tags are forced to <br/> in order to comply with Kindle XHTML requirements.

Closes #2670.

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I open the PR for curiosity first. The code works, but there are many issues with Kindling, among them:

1. Most of words do not work (like looking for "péri" that doesn't show up "périr" while this variant is part of the HTML file), and like 50-60% of words that are actually in the dictionary are not found by my Kindle.
2. HTML is stripped, meaning we loose our beautiful lists, and space between POS.
3. There are many warnings from the generated XHTML, I fixed a bunch already (81a343c, f97e892, a6459f0, 5d726f7) but we should try to fix most of them, e.g.:

[warning R6.10] section 6 (p.22): XHTML references a named entity that is not in the XML 1.0 predefined set or the common HTML5 whitelist. Undeclared entities render as literal text on Kindle. Entity:   (g000865.xhtml)`

On the positive side:

• The whole French dictionary is generated in 1 minute and 13 seconds (that's quite impressive!).
• Quite an interesting code clean-up on our side.

Still need to check:

• maximum variants count
• languages support

That's not acceptable as-is, and I try from time to time with newer Kindling version to see if improvements arise.

Latest Kindling version tested is v0.19.1.

[BoboTiG]

Update with v0.20.3: the primary problem (point 1) is now fixed ✔️

• Point 2 is still important to consider before doing the move.
• Point 3 is no blocker.

I'll test variants and languages support.

[BoboTiG]

According the its creator (ciscoriordan/kindling#8 (comment)) there is no maximum variants limit, and any language will work. This is very good!

[BoboTiG]

Point 2 is fixed in v0.22.1. We are good to go!