Regulator_Switching: add new symbol L7987

Regulator_Switching: add new symbol L7987 (https://www.st.com/resource/en/datasheet/l7987.pdf)

S4.2 is being slightly violated here (Pins should be grouped by function/Positive power pins should be placed at top of symbol). The rationale behind this is to match the pin positions in the datasheet, see the attached screenshot:

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All contributions to the kicad library must follow the KiCad library convention

Thanks for creating a pull request to contribute to the KiCad libraries! To speed up integration of your PR, please check the following items:

• 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
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