Regulator_linear: Add Diodes Inc. AP2138N

The AP2138 is a SOT-23-3 low dropout, low IQ fixed positive linear regulator.

It has a bigger brother (part of the same datasheet) the AP2139 which is not part of this commit.

https://www.diodes.com/assets/Datasheets/AP2138-9.pdf

• 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.