Adds a `prepublishOnly` script and standardizes some other package scripts per our module template.

This PR lowers the minimum Node version to 12.11.0 in the manifest `engines` field, and sets the version in the `.nvmrc` to `v12`.

We increased the minimum Node version to 14 in #77, unfortunately for unknown reasons (`:spiderman-pointing-gif:`). Ideally, we keep it working with all current LTS versions. Since `worker_threads` [first became stable](https://nodejs.org/en/blog/release/v12.11.0/) in 12.11.0, we can't go below that, but that's fine.

Closes #74.

This PR adds unit tests for `cli.ts`, and achieves 100% coverage for everything except branches, which are left at 86%. The lower branch coverage is mainly due to solitary `if` statements whose implicit `else` clauses are never tested.

In order to achieve the increased test coverage, some internal files have been refactored. All changes should be non-breaking.

The scripts in `development` were previously migrated to TypeScript. They were somewhat broken. This PR makes them plain JavaScript again, and removes them from the build output. This alters the structure of `dist`, which has been taken into account.

This also fixes a bug introduced in #126, due to `evalWorker.ts` being renamed to `eval-worker.ts`.

Following #129, updates the TypeScript config to more closely match the MetaMask module template.

* Bump @metamask/eslint-config from 6.0.0 to 8.0.0

Bumps [@metamask/eslint-config](https://github.com/MetaMask/eslint-config) from 6.0.0 to 8.0.0.
- [Release notes](https://github.com/MetaMask/eslint-config/releases)
- [Commits](MetaMask/eslint-config@v6.0.0...v8.0.0)

---
updated-dependencies:
- dependency-name: "@metamask/eslint-config"
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

* Manually fix lint issues, simplify config

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Erik Marks <rekmarks@protonmail.com>

Refactors the package exports (i.e. the exports of the package manifest `main` file, which is `dist/cmds/index`) such that the complete command array is exported under the name `commandModules`, and every individual command handler is exported under its name. The default export is removed.

This PR reverts #131, and completely removes the `main` field from the package manifest. We don't actually want to maintain any contract with consumers regarding our command module exports! The CLI itself is the interface. Packages that want to programmatically invoke `snaps-cli` can use `execa` or the command line.

This renames the `./development` directory to `./scripts`, which is more in line with other repositories in our organization.

This PR adds a mock `request` method and all of the `EventEmitter` methods to the mock `wallet` provider object in `eval-worker.ts`. These were previously missing, and made it fail example snaps that should succeed.