Add experimental.configFile setting

[bradlc]

Fixes #348

This PR adds a new experimental setting, tailwindCSS.experimental.configFile. This setting allows users to manually specify the Tailwind config file(s) that should be read to provide IntelliSense features.

For projects with a single Tailwind config file:

"tailwindCSS.experimental.configFile": ".config/tailwind.config.js"

For projects with multiple config files an object can be provided. Each key is a config file path and each value is a glob pattern (or array of patterns) representing the set of files that the config file applies to:

"tailwindCSS.experimental.configFile": {
  "themes/simple/tailwind.config.js": "themes/simple/**",
  "themes/neon/tailwind.config.js": "themes/neon/**"
}

Note that experimental settings may be changed or removed at any time.