logging: default to log level "error" instead of "debug"

[justinmk]

Problem

Currently we default to "debug" log level:

node-client/packages/neovim/src/utils/logger.ts

Line 3 in aac45ab

const level = process.env.NVIM_NODE_LOG_LEVEL || 'debug';

That seems a bit strange, it requires all consumers to undo that for production. Example:
https://github.com/vscode-neovim/vscode-neovim/blob/7592f1909467907da934e1748b1b7d5904dba67d/webpack.config.js#L50-L53

Expected behavior

Change default log-level. Update docs and possibly provide a warning during a "transitional" period.