fix: support the case of multiple line segments each with one vdim

[grapesmoker]

Previously Path would accept data that was structured as a list of dictionaries, each dictionary representing a single line segment. Such lists would plot correctly but if you had any vdims defined on the dictionary, a warning would be raised when trying to create a ColumnDataSource because the _element_transform function would make it so that the list of vdims and the list of coordinates had different lengths. This PR fixes this issue, so a list of dictionaries, each with its own coordinates and vdims, is now allowed. The below screenshot illustrates the desired plot.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.76%. Comparing base (becb5f3) to head (ba8c018).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6492   +/-   ##
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  Coverage   88.76%   88.76%           
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  Files         323      323           
  Lines       68696    68712   +16     
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+ Hits        60976    60992   +16     
  Misses       7720     7720           

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[hoxbro]

Thank you for the PR 👍