config: `configlet lint` warns for concepts

For example, even though every exercise will use functions, not every exercise should have `functions` in its `prerequisites`.

```text
The following concepts exist in the `practices` array of a Practice Exercise
in `/foo/exercism-tracks/zig/config.json`, but do not exist in the top-level `concepts` array
functions
slices
type-coercion
conditionals
builtin-functions
control-flow
bitwise-operations
importing
math
integers
error-sets
enums
arrays
pointers
methods
structs
optionals
allocators
algorithms
generics
data-structures
comptime

The Concept `control-flow` appears 11 times in the `practices` arrays of user-facing Practice Exercises,
but can only appear at most 10 times:
/foo/exercism-tracks/zig/config.json

The Concept `functions` appears 17 times in the `practices` arrays of user-facing Practice Exercises,
but can only appear at most 10 times:
/foo/exercism-tracks/zig/config.json

The Concept `conditionals` appears 12 times in the `practices` arrays of user-facing Practice Exercises,
but can only appear at most 10 times:
/foo/exercism-tracks/zig/config.json
```