Profile node: Interpolation command

[portnov]

This adds a new command, @I / @i, to the syntax of Profile Mk3 node definitions.

@I / @i command is an extension of SVG syntax. It defines a curve as
interpolating NURBS curve through the provided list of points. Current point,
at which the pen is located at the start of command, will be considered as
first point to be interpolated. If z is specified in the end, the curve will
be closed (cyclic); otherwise, this command will define an open curve. If
special keyword, @smooth, is specified, then, while computing interpolation
curve, this command will consider previous curve segment, defined by previous
command (C/c, S/s, Q/q, T/t), if it is of the same degree, and draw the curve
in such a way that in the starting point it will have the same tangent vector,
as previous curve had in the end point; in other words, at the meeting point of
segments, the curve will be smooth. Providing z and @smooth for the same
@i command is not supported.

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• Code changes complete.
• Code documentation complete.
• Documentation for users complete (or not required, if user never sees these changes).
• Manual testing done.
• Unit-tests implemented.
• Ready for merge.