fix: improve error handling for missing @payload-config path alias

[marwinhormiz]

Description

Currently, when the @payload-config path alias is missing from tsconfig.json, the package fails with a cryptic error when running commands such as generate:types or generate:importmap:

node:path:1464
    validateString(path, 'path');
    ^
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined

This happens because configPath becomes undefined when the path alias is missing, which later causes a path resolution error. This PR improves error handling to provide clearer feedback.

Changes

Added explicit error handling when @payload-config path alias is missing
Added descriptive error message to guide users on how to fix the issue
Added fallback to default configuration path (./src/payload.config.ts)

Related Issue

Closes #9796