Add a "line metric" expression operator to control styling along lines

[jahow]

This proposal aims at implementing a new line-metric operator which will return the "M" component (in a XYM layout) of points alongside a linestring geometry. If an M component is not available, actual distance in world coordinates will be used.

This can have several interesting use cases such as:

• Filtering parts of a line, e.g. based on time

lineStyle: {
  'stroke-width': 3,
  'stoke-color': 'blue',
  filter: ['&&',
    ['>', ['line-metric'], ['var', 'min']],
    ['<', ['line-metric'], ['var', 'max']],    
  ]
}

• Showing a gradient alongside a line

lineStyle: {
  'stroke-width': 3,
  'stoke-color': ['interpolate', ['linear'], ['line-metric'], 0, 'blue', 1, 'yellow']
}

The value will be interpolated linearly between two vertices of a LineString.

Proposal made in conjunction with @romprokop