Add walk as default value for first and last legs in PublicTransportMode

[Mind-the-Cap]

Add walk as default value for first and last legs in PublicTransportMode, as it will be easier for users

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (c3cea11) to head (59acadb).
⚠️ Report is 9 commits behind head on main.

❌ Your project check has failed because the head coverage (70.42%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
=======================================
  Coverage   70.42%   70.42%           
=======================================
  Files          56       56           
  Lines        2424     2424           
=======================================
  Hits         1707     1707           
  Misses        717      717           

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