Add ::MIME"text/plain" versions of show_special macros

[joschmitt]

Necessary for thofma/Hecke.jl#1370 (assuming we really want to go down this rabbit hole).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0ef1f6a) 87.17% compared to head (08cd804) 87.17%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1583      +/-   ##
==========================================
- Coverage   87.17%   87.17%   -0.01%     
==========================================
  Files         115      115              
  Lines       29565    29565              
==========================================
- Hits        25773    25772       -1     
- Misses       3792     3793       +1     

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[thofma]

I don't know all the ramifications, but this seem to not break anything, so let's get this in? @lgoettgens

[lgoettgens]

I have no idea how macros work. So this is up to you.
If this gets merged and released, it will be available in Hecke once thofma/Hecke.jl#1371 is merged there.

[thofma]

The macro doesn't do anything. It is just some little trick to be able to sneak in a return inside the caller.

I will merge this for now.