ja: fix the symbol readings, including the variable a being read as an eagle

[yasumorishima]

Same defect as #734, in the symbols rather than the accents: the seed translated
the English source word in its everyday sense, so several of these say something
unrelated. The matching entries in definitions.yaml are in #730; this PR is
the rule files.

Only strings change. No rule is added, removed, or restructured, and
audit-translations ja reports the same 28 rule differences as ja does.

The letter a was read as an eagle

unicode.yaml starts with a note to translators:

most languages don't have two ways to pronounce 'a' -- if not need, remove the
rules and change "b-z" to "a-z"

English needs the special case because the letter a and the article a sound
alike, so with no TTS it says "eigh". Japanese has no such ambiguity, and the
seed rendered "eigh" as イーグル — an eagle. Both the lowercase and the
uppercase rule had it, so every formula containing the variable a said "eagle".
It now says エー (and 大文字 エー when capital).

I kept the rule structure instead of deleting it as the note allows: removing it
would leave Missing rules: 2 in audit-translations permanently, and
# audit-ignore does not suppress that category. Say the word if you would
rather have the rules removed — the spoken result is the same either way.

The rest

symbol	English	Japanese was	which means	now
∥ 0x2225	parallel to	平行へ	a direction particle that cannot follow this noun	平行
∦ 0x2226	not parallel to	並行しない	並行 is concurrent, a different word from 平行	平行でない
≤ 0x2264	less than or equal to	より少しまたは等しい	"a little more, or equal"	より小さいか等しい
≥ 0x2265	greater than or equal to	より大きいか等しいへの	trailing particle, ungrammatical	より大きいか等しい
≡ 0x2261	identical to	同じ	"the same", weaker than the relation	恒等的に等しい
! 0x21	factorial	ファクシャル	not a word	階乗
! 0x21 (literal)	exclamation point	排除ポイント	"exclusion point"	感嘆符
∣	divides	分割する	to split a thing into parts, not divisibility	割り切る
pochhammer	permutations of	パーミュテーションの	transliteration	順列の

分割する appeared in both branches of the vertical-line rule, and 平行/並行 are
distinct words that a reader cannot tell apart by ear in context, so the
negative form was asserting something else entirely.

Tests

Five assertions in tests/Languages/ja/ja.rs: the letter a, the two parallel
relations, the two comparisons with equality, and a factorial.

The tests use x and y rather than a and b on purpose, so that they do not
depend on the letter-a rule this PR changes.