Add ability to specify a custom message for the ts-ignore plugin

[NickHeiner]

Related: #168.

This PR is likely easier to review commit-by-commit.

I moved the ts-ignore tests to be snapshot tests, since that makes it much easier to tweak the inserted message. I did the snapshot conversion as a separate commit, so you can verify the conversion itself didn't introduce any changes.

In addition to adding the ability to specify a custom message, I changed ts-migrate(${code}) to TS(${code}). This is more compact, and makes it more obvious that it's a TS-xxxx type error code.

[devinrhode2]

@NickHeiner how do I actually pass in this custom message?
I tried this but didn't seem to work:

yarn ts-migrate migrate . --plugin ts-ignore -p L1
yarn ts-migrate migrate . --plugin ts-ignore --p L1

[devinrhode2]

This seems to add a message prefix:

yarn ts-migrate reignore . --messagePrefix "level0" --stripIgnores true --eslintFixChanged=false

I wasn't exactly sure hot to specify the boolean flags though. I couldn't get the eslint-fix plugin to NOT run.

[NickHeiner]

I tried this but didn't seem to work:

Right – you were using the migrate command, but the p flag is defined on the reignore command.

I don't see eslintFixChanged as a flag in the CLI setup, which explains why that flag doesn't do anything when you pass it. 😄