neovim fix

[kbrose]

Vim packages, man.

Checklist

• [x] Relevant issues have been referenced
• [x] CHANGELOG.md has been updated (bullet points added to the Unreleased section)
• [x] Tests have been added, or are not relevant (not automatically testable, unfortunately)

Description

Hopefully fixes #82

Adds another when context to package.json that only runs the extension when neovim is in the "insert" mode.

Fix README issue that I noticed a while ago, too.