PEP 639: Make requested additions and changes, add additional helpful…

… info, reformat, update and copyedit (#2164)

* PEP 639: Reframe core metadata update as discrete proposal per PEP 643

* PEP 639: Clean up formatting, syntax, spelling, headers and lists

* PEP 639: Rewrite to reflect License-Expression field and depr process

* PEP 639: Update examples, current tools survey, links and more

* PEP 639: Add PEP 621, expand/move examples & reorganize/shorten headings

* PEP 639: Further specify and clarify license file handling

* PEP 639: Add additional ambigous classifiers and tool recommendations

* PEP 639: Rework PEP 621 keys, refine dynamic/globs & add rejected ideas

* PEP 639: Specify license path root & subdirs, & expand examples

* PEP 639: Add license_files dir, rejected ideas & sdist/wheel/installed

* PEP 639: Add PyPI validation of new fields for newly-uploaded files

* PEP 639: Add open isssue for back-filling & reject per-dist licenses

* PEP 639: Add User Scenarios to provide guidance for common use cases

* PEP 639: Update Abstract, Goals, Rationale, Compat. & other sections

* PEP 639: Add terminology section & use consistant, clear & correct terms

* PEP 639: Make refs inline per #2130, add links, fix others & refine text

* PEP 639: Copyedit and proofread for grammar, phrasing, clarity & tone

* PEP 639: Address reviewer and community feedback

* PEP 639: Add custom IDs issue & clarify rejected license key str value