DFN/QFN generator: lead width/length can be different on h/v sides

This adds a way to specific the lead width and length separately for the horizontal and vertical sides of the footprint.

It includes the fix mentioned in !496 (closed) as this now becomes important.

Horizontal leads (those on the vertical sides) are defined by 'lead_len_H' and 'lead_width_H', and vertical leads by '*_V'.

Pitches in the x and y directions are 'pitch_x' and 'pitch_y'.

If these aren't given, the values for 'lead_len', 'lead_width' and 'pitch' are used for all leads, so all current definitions remain unchanged.

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The motivation was to implement the TI RUX0012A part (kicad-footprints!1422 (closed)), but actually using the datasheet tolerances means it doesn't quite come out the same and the thermal vias are not supported.

But you can still try it:

# RUX0012A
Texas_VQFN-HR-12_2.0mmx2.5mm:
  device_type: 'QFN'
  manufacturer: 'Texas'
  #part_number: 'mpn'
  size_source: 'https://www.ti.com/lit/ml/mpqf508/mpqf508.pdf'
  ipc_class: 'qfn' # 'qfn_pull_back'
  #ipc_density: 'least' #overwrite global value for this device.
  custom_name_format: 'Texas_VQFN-{pincount}'
  body_size_x:
    minimum: 1.9
    maximum: 2.1
  body_size_y:
    minimum: 2.4
    maximum: 2.6
  overall_height:
    maximum: 1
  lead_width_V:
    minimum: 0.15
    maximum: 0.25
  lead_len_V:
    minimum: 0.3
    maximum: 0.5
  lead_width_H:
    minimum: 0.25
    maximum: 0.45
  lead_len_H:
    minimum: 0.75
    maximum: 0.95

  pitch_x: 0.5
  pitch_y: 0.7
  num_pins_x: 4
  num_pins_y: 2