unexpand: add support for extended tab stop syntax (+N and /N) #16882

Job	Run time
Build the fuzzers	1m 8s
Build and test uufuzz examples	1m 8s
Fuzz (fuzz_echo, true)	3m 31s
Fuzz (fuzz_date, true)	3m 32s
Fuzz (fuzz_printf, true)	2m 8s
Fuzz (fuzz_test, true)	2m 10s
Fuzz (fuzz_expr, true)	3m 31s
Fuzz (fuzz_sort, false)	3m 14s
Fuzz (fuzz_env, false)	1m 7s
Fuzz (fuzz_seq_parse_number, true)	1m 1s
Fuzz (fuzz_parse_size, true)	1m 3s
Fuzz (fuzz_cut, false)	1m 4s
Fuzz (fuzz_tr, false)	1m 33s
Fuzz (fuzz_seq, false)	1m 4s
Fuzz (fuzz_cksum, false)	1m 10s
Fuzz (fuzz_parse_time, true)	1m 36s
Fuzz (fuzz_split, false)	3m 30s
Fuzz (fuzz_wc, false)	1m 7s
Fuzz (fuzz_parse_glob, true)	2m 6s
Fuzz (fuzz_non_utf8_paths, true)	2m 3s
Fuzzing Summary	7s
	38m 53s

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