dist should ignore non-.yml files

[autonome]

/Users/dietrich/misc/web-features/scripts/dist.ts:378
throw new Error(
^

Error: Cannot generate dist for features/draft/spec/.shared-storage.yml.swp, only YAML input is supported
at (/Users/dietrich/misc/web-features/scripts/dist.ts:378:15)
at Array.map ()
at main (/Users/dietrich/misc/web-features/scripts/dist.ts:370:15)
at (/Users/dietrich/misc/web-features/scripts/dist.ts:421:3)
at ModuleJob.run (node:internal/modules/esm/module_job:218:25)
at async ModuleLoader.import (node:internal/modules/esm/loader:323:24)
at async loadESM (node:internal/process/esm_loader:28:7)
at async handleMainPromise (node:internal/modules/run_main:120:12)

[ddbeck]

This is due to the change in #1775. It really ought to catch specific mistakes we know about instead of catching any unrecognized file extension.