spelling: don't consider digits as word parts, because GNU spell doesn't

This fixes https://savannah.gnu.org/bugs/?48660.

src/chars.c

@@ -93,6 +93,26 @@ void wctomb_reset(void)
     IGNORE_CALL_RESULT(wctomb(NULL, 0));
 }
 
+/* This function is equivalent to isalpha() for multibyte characters. */
+bool is_alpha_mbchar(const char *c)
+{
+    assert(c != NULL);
+
+#ifdef ENABLE_UTF8
+    if (use_utf8) {
+	wchar_t wc;
+
+	if (mbtowc(&wc, c, MB_CUR_MAX) < 0) {
+	    mbtowc_reset();
+	    return 0;
+	}
+
+	return iswalpha(wc);
+    } else
+#endif
+	return isalpha((unsigned char)*c);
+}
+
 /* This function is equivalent to isalnum() for multibyte characters. */
 bool is_alnum_mbchar(const char *c)
 {

src/proto.h

@@ -183,6 +183,7 @@ bool nisblank(int c);
 bool niswblank(wchar_t wc);
 #endif
 bool is_byte(int c);
+bool is_alpha_mbchar(const char *c);
 bool is_alnum_mbchar(const char *c);
 bool is_blank_mbchar(const char *c);
 bool is_ascii_cntrl_char(int c);

src/utils.c

@@ -290,12 +290,11 @@ bool is_separate_word(size_t position, size_t length, const char *buf)
     parse_mbchar(buf + move_mbleft(buf, position), before, NULL);
     parse_mbchar(buf + word_end, after, NULL);
 
-    /* If we're at the beginning of the line or the character before the
-     * word isn't a non-punctuation "word" character, and if we're at
-     * the end of the line or the character after the word isn't a
-     * non-punctuation "word" character, we have a whole word. */
-    retval = (position == 0 || !is_alnum_mbchar(before)) &&
-		(word_end == strlen(buf) || !is_alnum_mbchar(after));
+    /* If the word starts at the beginning of the line OR the character before
+     * the word isn't a letter, and if the word ends at the end of the line OR
+     * the character after the word isn't a letter, we have a whole word. */
+    retval = (position == 0 || !is_alpha_mbchar(before)) &&
+		(word_end == strlen(buf) || !is_alpha_mbchar(after));
 
     free(before);
     free(after);