feat:system: Add system utility library with executable check

Introduce a new `system-lib.el` module providing low-level system
utility functions, including function `cj/executable-exists-p` to
check for the availability of programs in PATH. Integrate this
library in `init.el`.

test(system): Add unit tests for executable check function

Create comprehensive unit tests for `cj/executable-exists-p` in
`system-lib.el`, ensuring coverage of normal, boundary and error
scenarios.

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