Update wasm dependencies

[ianthomas23]

This PR updates the dependencies used for WebAssembly testing. xtermjs goes from 5 to 6, in line with the latest in cockle and the fontSize: 14 is part of this.

It also addresses vulnerability https://github.com/QuantStack/git2cpp/security/dependabot/2 by updating deepmerge-ts, even though it is not a real-world problem for us.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (c947fbf) to head (63ff7a6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #185   +/-   ##
=======================================
  Coverage   88.22%   88.22%           
=======================================
  Files          63       63           
  Lines        2997     2997           
  Branches      376      376           
=======================================
  Hits         2644     2644           
  Misses        353      353           

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[ianthomas23]

Wasm tests pass https://github.com/QuantStack/git2cpp/actions/runs/35198599146/job/105127644144