Add mechanical limit switch symbol

Adding a symbol for a mechanical limit switch SPDT using the NEMA symbol, I made this MR because the push button SPDT and the mechanical endstop SPDT symbols and the numbering are slightly different, also I have created 3 limit switch footprints & 3D models to use with along this symbol.

• I created a MR for some footprints to use with this symbol: kicad-footprints!2897
• I created a MR for the source of 3D models: kicad-packages3D-source!346
• I created a MR for the step & wrl files: kicad-packages3D!818

• Remove the instructional content of this template
• Provide a URL to a datasheet for the symbol(s) you are contributing
• An example screenshot image is very helpful
• Ensure that the associated footprints match the official footprint library
  • A new fitting footprint must be submitted if the library does not yet contain one.
• If there are matching footprint PRs, provide link(s) as appropriate
• Check the output of the automated check scripts - fix any errors as required
• Give a reason behind any intentional library convention rule violation.