Warn about invalid globs in content

[RobinMalfait]

This PR will try and fix bogus globs

When using globs, there is this option called "brace expansion", this means that you can simplify some globs instead of repeating the same thing over and over again. For example:

./src/**/*.html
./src/**/*.js

Can be simplified as:

./src/**/*.{html,js}

This is also what we explain in our docs: https://tailwindcss.com/docs/installation

However, it seems like some people are changing that glob to better match their project which is totally fine. However, a lot of people end up with invalid globs.

./src/**/*.{html}

This is an invalid glob, notice that there is only 1 item inside the { & }.
This is the same behaviour as brace expansion in your terminal.

Here is an example from my terminal output inside the tailwindcss repo:

$ ls -la *.{json} # Invalid
zsh: no matches found: *.{json}

$ ls -lah *.json # Valid
-rw-r--r--  1 robin  staff   438K Dec 12 22:18 package-lock.json
-rw-r--r--  1 robin  staff   3.1K Dec 12 22:18 package.json

$ ls -lah *.{json,js} # Valid
-rw-r--r--  1 robin  staff   120B Dec 12 22:18 colors.js
-rw-r--r--  1 robin  staff   156B Dec 12 22:18 defaultConfig.js
-rw-r--r--  1 robin  staff   151B Dec 12 22:18 defaultTheme.js
-rw-r--r--  1 robin  staff   438K Dec 12 22:18 package-lock.json
-rw-r--r--  1 robin  staff   3.1K Dec 12 22:18 package.json
-rw-r--r--  1 robin  staff   151B Dec 12 22:18 plugin.js
-rw-r--r--  1 robin  staff   446B Sep  1 17:36 prettier.config.js
-rw-r--r--  1 robin  staff   156B Dec 12 22:18 resolveConfig.js

This PR will try and detect those bogus cases and rewrite show a warning with a fix for the glob.

./src/**/*.{html}

Should become:

./src/**/*.html

Example warning output:

warn - The glob pattern ./src/**/*.{html} in your config is invalid.
warn -     Update it to ./src/**/*.html to silence this warning.

Fixes: #6394