k4Const: prevent kVarName-like variables from modifying it.

Example

go install github.com/lance6716/k4Const/cmd/k4const@latest
go vet -vettool=$(which k4const) ./...

Supported pattern

See test

Rationale

1. When the codebase is big, I really miss the const type qualifier to prevent unexpected modification.
2. Golang uses identifier names to represent public/private, so it's similar to use it for other purpose.
3. It's a general notation to use k in naming, to represent it's a constant.