Add `3x5_3.h` etc aliases for `36.h` etc

[urob]

Labeling layouts by the number of keys is not very informative. It would be more descriptive to use something along the following lines:

• 4x6.h, 5x6.h etc for rectangular layouts such as Plank
• 3x5_3.h for rectangular layouts with a single thumb column such as Sweep or Corne
• Custom header names based on a popular representative for anything more complex, e.g., hummingbird.h

For backward compatibility, aliases should be added for existing headers.

[lazyzyf]

good idea.