[SCHEMA] Add extensions to schema

[tsalo]

Closes #1035.

Changes proposed:

• Add a YAML file to schema/objects/ with definitions of file extensions.
• Add valid extensions that were missing for meg and headshape files.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.50%. Comparing base (4a6c49a) to head (7e94f22).
Report is 1460 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1036       +/-   ##
===========================================
+ Coverage   34.05%   71.50%   +37.45%     
===========================================
  Files           8        9        +1     
  Lines         834      930       +96     
===========================================
+ Hits          284      665      +381     
+ Misses        550      265      -285     

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[tsalo]

The following suffixes are actually missing from the schema:

• .trg
• .md
• .pos
• .rst
• .chn
• .txt

I think txt, rst, and md are all added for the first time in #1033, but that means trg, pos, and chn are missing.
EDIT: Added in 0acbfbe

[sappelhoff]

apart from my one comment, this lgtm