Add JEITA SC-74A (5 pin variant of SC-74)

This adds the JEITA SC-74A package, which is a 5-pin variant of the SC-74 package (recently standardised to JEITA values in !1005 (merged))

https://www.jeita.or.jp/japanese/standard/book/ED-7500B/#target/page_no=41

This is a package with many names according to manufacturers, such as SOT-25 and SOT-23-6 (often these packages are manufacturers-specific tightening of the JEITA SC-74A tolerances.

This is the SC-74A over the SC-74 (flipped, blue) - everything is the same except the missing pad:

Related: kicad-footprints!3240 (closed) (which is a Diodes Inc. SOT-25, which is a "tightening" of this generic part (which should fit any SC-74A, not just Diodes parts).

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To do this, the IPC Gullwing generator got two new features in separate commits:

• YAML inheritance to allow derivation of related parts (will also be useful elsewhere)
• Override of the pin_count so that the name can be SC-74A-5...., not SC-74A-6-5... as would otherwise be the default.

The Readme was updated with these features.