feat: support groups

[nrwiersma]

Goal of this PR

This adds support for groups, allows group prefixing to be simpler.

The performance change is as follows, and acceptable:

name                   old time/op    new time/op    delta
Logger_Logfmt-8          9.38ns ±35%   10.74ns ±10%     ~     (p=0.133 n=10+9)
Logger_Json-8            11.3ns ±39%    11.5ns ±40%     ~     (p=0.896 n=10+10)
Logger_LogfmtWriter-8    28.8ns ±20%    36.0ns ±14%  +25.14%  (p=0.000 n=10+10)
Logger_LogfmtWithTS-8    14.0ns ±19%    13.3ns ±14%     ~     (p=0.412 n=10+9)
Logger_JsonWithTS-8      14.2ns ±27%    14.5ns ±20%     ~     (p=0.796 n=10+10)
Logger_LogfmtCtx-8       23.4ns ± 8%    23.1ns ±18%     ~     (p=0.549 n=9+10)
Logger_JsonCtx-8         26.3ns ±25%    27.1ns ±14%     ~     (p=0.356 n=10+9)
LevelLogger_Logfmt-8     91.0ns ±14%    89.7ns ±25%     ~     (p=0.781 n=10+10)
LevelLogger_Json-8       94.7ns ±14%    93.5ns ±25%     ~     (p=0.704 n=9+10)

name                   old alloc/op   new alloc/op   delta
Logger_Logfmt-8           0.00B          0.00B          ~     (all equal)
Logger_Json-8             0.00B          0.00B          ~     (all equal)
Logger_LogfmtWriter-8     16.0B ± 0%     16.0B ± 0%     ~     (all equal)
Logger_LogfmtWithTS-8     0.00B          0.00B          ~     (all equal)
Logger_JsonWithTS-8       0.00B          0.00B          ~     (all equal)
Logger_LogfmtCtx-8        0.00B          0.00B          ~     (all equal)
Logger_JsonCtx-8          0.00B          0.00B          ~     (all equal)

name                   old allocs/op  new allocs/op  delta
Logger_Logfmt-8            0.00           0.00          ~     (all equal)
Logger_Json-8              0.00           0.00          ~     (all equal)
Logger_LogfmtWriter-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
Logger_LogfmtWithTS-8      0.00           0.00          ~     (all equal)
Logger_JsonWithTS-8        0.00           0.00          ~     (all equal)
Logger_LogfmtCtx-8         0.00           0.00          ~     (all equal)
Logger_JsonCtx-8           0.00           0.00          ~     (all equal)

How did I test it?

Unit tests.