feat: add warning spec attribute

[vergiLgood1]

Summary

Adds an optional warning message to validator specs. When an env var is not set and a fallback value (default, devDefault, or testDefault) is used, the message is surfaced by the reporter without failing the program.

Closes #220

Design decision

The issue's example uses str({ warning: "..." }) with no default and implies the app should keep running ("App will run but it will not send any email"). However, making warning implicitly turn a required var into an optional one would conflict with the library's philosophy (a program should only run when its environment dependencies are met) and could silently weaken validation — e.g. someone adding a warning message to a required database password var.

So this PR keeps optionality explicit, consistent with how default: undefined already works:

const env = cleanEnv(process.env, {
  EMAIL_SMTP: str({
    default: undefined,
    warning: 'EMAIL_SMTP is not set. App will run but it will not send any email.',
  }),
})
// Program runs, value is undefined, and the warning is logged.

With a concrete fallback, the warning fires when that fallback is used:

const env = cleanEnv(process.env, {
  RETRIES: str({ default: '3', warning: 'RETRIES not set, using default 3' }),
})

I'm happy to adjust if a different behavior (e.g. warning implying optionality) is preferred.

Changes

• src/types.ts — new warning?: string on Spec<T>; new optional warnings field on ReporterOptions<T>
• src/core.ts — getSanitizedEnv records a warning whenever the env var was not set and a fallback (testDefault / devDefault / default) was used, and passes warnings to the reporter. With reporter: null the caller opts out of all reporting, including warnings (errors are still thrown, as before)
• src/reporter.ts — defaultReporter prints a Warnings: section (same visual style as the error sections). Warnings never affect the exit behavior: with only warnings the process exits normally; with errors the existing error/exit flow is unchanged
• README.md — documents the new attribute
• tests/warning.test.ts — new: warning recorded for default / devDefault / testDefault fallbacks, no warning when the var is set, missing var without any fallback still errors, reporter: null behavior, and type-level assertions (optionality still driven by default: undefined, not by warning)
• tests/reporter.test.ts — new: warnings printed without exiting, warnings + errors printed together with exit preserved, warnings sorted
• tests/requiredWhen.test.ts — updated to match the new reporter call signature (warnings: {})

Verification

• bun run ts ✅
• bun run lint ✅
• bun run test:ci ✅ (82 tests, 8 files)
• bun run coverage ✅ (functions 100%, lines 98.89% — above the 98% thresholds)

Example output:

 =================================
  Warnings:
     EMAIL_SMTP: EMAIL_SMTP is not set. App will run but it will not send any email.
 =================================