feat: Add `inputOnly` option for exclusive input utilization

[snaka]

This commit adds the inputOnly option to EnvsafeOpts, enabling exclusive use of input values and enhancing typo detection. When inputOnly is set to true, envsafe disregards runtime environment variable references and solely relies on the provided input values.

This improvement ensures better reliability and prevents unnoticed typos in environment variable references that were previously masked by fallback values.

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I strongly agree with the opinion that there could be a better name than "inputOnly" for the introduced option.

Closes #146