Improve error message truncation with closing delimiters

[itchyny]

Truncated strings now show closing delimiters: ..." for strings,
...] for arrays, and ...} for objects, making error messages clearer.

[itchyny]

I think the error buffer 15 is too small. I'd like to increase to 20. WDYT?

[wader]

I think the error buffer 15 is too small. I'd like to increase to 20. WDYT?

There are some 30 used with jv_dump_string_trunc already, maybe increase all to that?

$ git grep -E '(key|err)buf.*\[.*\]' | cat
src/builtin.c:  char errbuf[15];
src/builtin.c:  char errbuf1[15],errbuf2[15];
src/execute.c:        char errbuf[15];
src/execute.c:        char errbuf[30];
src/execute.c:        char keybuf[15];
src/execute.c:        char errbuf[30];
src/execute.c:          char errbuf[15];
src/parser.c:    char errbuf[15];
src/parser.y:    char errbuf[15];