[pt] Improve behaviour of compound measurements

[p-goulart]

For measurements that include multiplication signs, our disambiguator is not adding an ignore_spelling flag, which results in stuff like this:

From the speller dashboard:

^{Note that here they look like proper multiplication signs, but if you look at the regex and inspect the values, you'll see they are really just x. Probably the browser being clever when rendering stuff.}

---

This PR:

• adds X_AS_MULTIPLICATION_SIGN_IGNORE to the disambiguator (this the most important thing);
• adds tests to the speller and tokeniser (just to be on the safe side);
• add an example to MULTIPLICATION_SIGN picky rule (to make sure the x can properly be changed to a × even before unspaced units);
• improves the messages for UNITS_OF_MEASURE_SPACING picky rule (chapter 7,103 of 'our messages all sound like pt-PT).

Note that, because of our current tokenisation logic, unit spacing will not be fixed for tokens like 500x500mm. For it to work, one would first need to apply the MULTIPLICATION_SIGN suggestion. (IMO that's probably fine, since these are picky rules. The most crucial thing here was no longer flagging stuff like 5x5 and 5x5mm as spelling errors.)

[p-goulart]

There was a bit of a peak this week, but note how this is nothing new.

[p-goulart]

4de7888 adds DICE_ROLL_NOTATION to ignore stuff like 1d6 (roll one 6-sided die), 3d20 (roll three 20-sided dice), etc.