Fix CAPHI rewrite for stem-final i_y / u_w with no suffix

[AbdulmalikDS]

Fixes #138

rewrite_caphi rules 3 and 4 require a + after the stem marker, so the $
branch never matches. A stem ending in i_y or u_w with no suffix is left
as a short vowel + glide instead of a long vowel:

قاضِي   q_aa_d._i_y-   ->  q_aa_d._i_y    expected q_aa_d._ii
يَدْعُو  y_a_d_3_u_w-   ->  y_a_d_3_u_w    expected y_a_d_3_uu

The rules were right when written (027ec90, 2018): the caphi join always
emitted a +, even for an empty suffix. d99bbbc (2020) made the join skip
empty parts, which removed that trailing + and silently broke the no-suffix
case.

Moved \+ inside the group, as proposed in the issue. Same change was part of
#166, split out here on its own with tests.

Checked against calima-msa-r13: 3,062 stems end in i_y- / u_w-. Running
the analyzer on 400 of them (2,679 analyses), the only differences are
_i_y -> _ii (580) and _u_w -> _uu (104). Vowel-suffix forms like قاضِيَكَ
are unchanged.