refactor(completion): expression based variable/cell_path completion #27

Job	Run time
tests (windows-latest)	6m 48s
std-lib-and-python-virtualenv (ubuntu-20.04, py)	2m 18s
plugins (windows-latest)	4m 2s
tests (macos-latest)	3m 4s
std-lib-and-python-virtualenv (macos-latest, py)	2m 58s
fmt-clippy (windows-latest)	4m 31s
plugins (macos-13)	3m 13s
tests (ubuntu-20.04)	4m 17s
fmt-clippy (macos-13)	2m 34s
std-lib-and-python-virtualenv (windows-latest, py)	8m 24s
plugins (ubuntu-20.04)	1m 52s
fmt-clippy (ubuntu-20.04)	1m 27s
Build WASM	1m 4s
Clippy WASM	1m 38s
	48m 10s

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