data(kana): Mozc's romaji table

hira.map and kata.map knew 152 keys of the 320 every Japanese IME
accepts: no nn, no small kana by x or l (xtu, xya, la, ...), no sya, tya,
zya, jya, dya, cya, tsa, thi, dhi, twu, kye, ye, fya, wha, and zi gave ぢ.
Both are now Mozc's default table in full, kata.map in Katakana, so that
they agree (kata.map alone had di → ディ and a lone v).  With Mozc, wi
and we are うぃ and うぇ (wyi and wye the old kana), the v row is ゔ, nn
is ん, and [ ] ~ and the z-prefixed symbols type 「 」 〜 ・ … ← ↓ ↑ →.

n' is a row now, not a case in transjp.  A row whose value is っ keeps
its consonant pending only when it is a doubled consonant or tch: xtu
and ltu are っ itself, so a lone っ can finally be typed.

map/README

@@ -38,6 +38,14 @@ are omitted.  The import preserves the upstream license header and row order.
 The retained data is BSD 3-Clause licensed by Choe Hwanjin; the complete text
 is in `LICENSES/BSD-3-Clause-libhangul-hanja.txt`.
 
+## Japanese romaji
+
+`hira.map` and `kata.map` are Mozc's default romaji table, one row per key,
+in gojūon order; `kata.map` is the same table in Katakana, and
+`make verify-map` checks that the two have the same keys.  A doubled
+consonant, or `tch`, maps to っ and keeps the consonant pending; the engine
+knows that rule, the table only lists the rows.
+
 ## Japanese dictionary data
 
 `kanji.dict` is the historical dictionary bundled with strans.  Its own header