Handle cut-off UTF-8 sequences when reading files

Read additional bytes from the file to complete the UTF-8 sequence so the bytes
in it don't get converted to U+FFFD replacement characters.

src/jv_unicode.h

@@ -1,6 +1,7 @@
 #ifndef JV_UNICODE_H
 #define JV_UNICODE_H
 
+const char* jvp_utf8_backtrack(const char* start, const char* min, int *missing_bytes);
 const char* jvp_utf8_next(const char* in, const char* end, int* codepoint);
 int jvp_utf8_is_valid(const char* in, const char* end);