Attempt to auto-detect configuration files with varying file extensions

[JamesMGreene]

Context

• Fixes Supported configuration file extension check does not use the per-generator default #104
• Fixes Doesn't recognize next.config.mjs #107

Purpose

If no generator file path is manually provided as input, this change loops through our supported file extensions seeking the existence of a configuration file that matches the static site generator's chosen configuration file basename, e.g. for Next.js:

• Does next.config.js exist?
• If not, does next.config.cjs exist?
• If not, does next.config.mjs exist?
• If not, fallback to next.config.js and create the file

Risks

I did not add any new tests for this slight behavioral change as it basically boils down to a loop that checks for file existence. Feel free to push back if you feel more testing is needed. 😅