Fix Tailwind config file resolution when Prettier config file is not present

[bradlc]

This PR fixes Tailwind config file resolution when no Prettier config file is present. Now, when the tailwindConfig option is not provided the plugin will search up the directory hierarchy from a "base" directory, looking for a config file. The base directory is determined in this order:

1. The directory containing the Prettier config file if there is one
2. The directory containing the current file (options.filepath), if provided
3. The current working directory (process.cwd())

Additionally, the VSCODE_CWD environment variable is no longer used because its value is not necessarily equal to the directory that is currently open in VS Code, as originally thought.