Fix time attribute formatting to match slog.TextHandler

[lmittmann]

This PR fixes an inconsistency in how time values are formatted. tint is meant to match the behavior of slog.TextHandler for log attributes beyond the standard attributes (slog.TimeKey...).

Performance Improvement

                                      │    v1.1.0    │                 pr                  │
                                      │    sec/op    │   sec/op     vs base                │
LogAttrs/h=tint/5_args-8                1282.0n ± 1%   949.0n ± 1%  -25.97% (p=0.000 n=10)
LogAttrs/h=tint/5_args_custom_level-8   1292.0n ± 2%   962.0n ± 1%  -25.54% (p=0.000 n=10)
LogAttrs/h=tint/10_args-8                2.293µ ± 2%   1.628µ ± 2%  -28.99% (p=0.000 n=10)
LogAttrs/h=tint/40_args-8                7.845µ ± 2%   5.035µ ± 2%  -35.82% (p=0.000 n=10)
LogAttrs/h=tint/error-8                  592.2n ± 1%   590.1n ± 2%        ~ (p=0.896 n=10)
LogAttrs/h=tint/attr-8                   536.9n ± 1%   537.9n ± 2%        ~ (p=0.542 n=10)
geomean                                  1.455µ        1.155µ       -20.59%

                                      │    v1.1.0    │                   pr                   │
                                      │     B/op     │     B/op      vs base                  │
LogAttrs/h=tint/5_args-8                 84.000 ± 0%     4.000 ± 0%  -95.24% (p=0.000 n=10)
LogAttrs/h=tint/5_args_custom_level-8    84.000 ± 0%     4.000 ± 0%  -95.24% (p=0.000 n=10)
LogAttrs/h=tint/10_args-8                 376.0 ± 0%     216.0 ± 0%  -42.55% (p=0.000 n=10)
LogAttrs/h=tint/40_args-8               2.032Ki ± 0%   1.407Ki ± 0%  -30.75% (p=0.000 n=10)
LogAttrs/h=tint/error-8                   36.00 ± 0%     36.00 ± 0%        ~ (p=1.000 n=10) ¹
LogAttrs/h=tint/attr-8                    32.00 ± 0%     32.00 ± 0%        ~ (p=1.000 n=10) ¹
geomean                                   136.1          42.31       -68.92%
¹ all samples are equal

                                      │   v1.1.0    │                  pr                  │
                                      │  allocs/op  │ allocs/op   vs base                  │
LogAttrs/h=tint/5_args-8                 3.000 ± 0%   1.000 ± 0%  -66.67% (p=0.000 n=10)
LogAttrs/h=tint/5_args_custom_level-8    3.000 ± 0%   1.000 ± 0%  -66.67% (p=0.000 n=10)
LogAttrs/h=tint/10_args-8                7.000 ± 0%   3.000 ± 0%  -57.14% (p=0.000 n=10)
LogAttrs/h=tint/40_args-8               25.000 ± 0%   9.000 ± 0%  -64.00% (p=0.000 n=10)
LogAttrs/h=tint/error-8                  2.000 ± 0%   2.000 ± 0%        ~ (p=1.000 n=10) ¹
LogAttrs/h=tint/attr-8                   1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=10) ¹
geomean                                  3.829        1.944       -49.22%