Make sure AsciiMath text elements are of kind #text

[dpvc]

This makes the legacy AsciiMath internal format to v3/v4 internal format properly mark the text elements as #text rather than text.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (f6bac7c).
Report is 36 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1306     +/-   ##
==========================================
  Coverage    86.71%   86.71%             
==========================================
  Files          337      337             
  Lines        83979    83979             
  Branches      4750     3124   -1626     
==========================================
  Hits         72826    72826             
- Misses       11130    11153     +23     
+ Partials        23        0     -23     

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[zorkow]

lgtm.