Add support for forcing colors in common/log

[perillo]

Please read the following before submitting:

• We are not accepting any new window management features unless they get implemented by i3.

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• Description:

  Currently the sway log system enables colors when stderr is attached to a tty.

  Unfortunately, the current implementation does not support colors when the the user wants to redirect the log output to both the terminal and a file.

  I propose to implement either http://bixense.com/clicolors/ or https://force-color.org/ standards.
  As a reference, the former is used by Zig (when generating the Zig Language Reference) and the latter is used by Python.

• Complexity

  The complexity of the feature should be low.

  There is one possible issue in https://github.com/swaywm/sway/blob/master/common/log.c, where isatty is called multiple times, instead of detecting the tty configuration in the sway_log_init (as it is done in https://github.com/ziglang/zig/blob/master/lib/std/io/tty.zig#L12).

  Thanks.