This constant is set to the
wezterm version string that is also reported
wezterm -V. This can potentially be used to adjust configuration
according to the installed version.
The version string looks like
20200406-151651-5b700e4. You can compare the
strings lexicographically if you wish to test whether a given version is newer
than another; the first component is the date on which the release was made,
the second component is the time and the final component is a git hash.