Build the embedded word lists lazily

[espadolini]

Maps in vars are created at init time, resulting in something like 2.5MiB of allocation at runtime that can be problematic for programs that only optionally use the generator; this PR changes the word lists from a literal map to an explicit function that does the assignment plus a sync.Once. I didn't use sync.OnceValue to keep the minimum Go requirement low.

The changes should be only a handful of lines on each file if you ignore whitespace in the diff view.

[sethvargo]

I actually think bumping the Go version and using sync.OnceValue is cleaner and more appropriate here: #12