internal placeholder plugin (#125)

* internal placeholder plugin

* fix panic on placeholder editor input

* remove autofocus in placeholder example

* make `Placeholder.active` private

* remove placeholder on input

* fix multi-byte char in placeholder

* show placeholder on empty editor

* add guards to placeholder add fns

* fix placeholder displaying incorrectly on input

* fix flash when backspacing empty placeholder

also properly fix the first-char display error

* hacky fix for delete key breaking placeholder

* fix newline issues in placeholder

* fix clippy

* update changelog, bump version

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Co-authored-by: StaffEngineer <111751109+StaffEngineer@users.noreply.github.com>
Co-authored-by: StaffEngineer <velo.app1@gmail.com>