feat: allow RandomPoints.start_at to be a position

[tlambert03]

this allows the start_at field of RandomPoints to be an actual position, with semantics:

    start_at : int | RelativePosition
        Position or index of the point to start at. This is only used if `order` is
        'nearest_neighbor' or 'two_opt'.  If a position is provided, it will *always*
        be included in the list of points. If an index is provided, it must be less than
        the number of points, and corresponds to the index of the (randomly generated)
        points; this likely only makes sense when `random_seed` is provided.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.76%. Comparing base (b623130) to head (4f01d17).

Files	Patch %	Lines
src/useq/_grid.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
- Coverage   93.97%   93.76%   -0.22%     
==========================================
  Files          18       18              
  Lines        1179     1187       +8     
==========================================
+ Hits         1108     1113       +5     
- Misses         71       74       +3     

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