Add manualpathconcat linter: flag manual "/" path concatenation (#54307)

* Initial plan

* Add manualpathconcat linter flagging manual "/" path concatenation

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Clarify separator-only left operand comment in manualpathconcat

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Add draft ADR-54307 for manualpathconcat linter

* Fix manualpathconcat chain suggestion fallback

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

* Add edge-case tests for manualpathconcat diagnostics

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Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
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Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>