Add VERSION file with current libtrixi version

[sloede]

Since we do not have a top-level Project.toml as in pure Julia projects, I suggest we add a VERSION file that contains the current semver version string and that can then be used to, e.g., populate tags etc.

Julia does something similar with https://github.com/JuliaLang/julia/blob/master/VERSION

[codecov]

Codecov Report

Merging #56 (141a8b3) into main (5b7bf84) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #56   +/-   ##
=======================================
  Coverage   70.83%   70.83%           
=======================================
  Files           5        5           
  Lines         168      168           
=======================================
  Hits          119      119           
  Misses         49       49           

Flag	Coverage Δ
unittests	70.83% <ø> (ø)

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