esm: Unflag --experimental-modules

This PR unflags the --experimental-modules support making modules on-by-default, while remaining backwards-compatible with the current runMain.

This PR should only land after the remaining PRs have been fully considered:

• Fix 3 remaining task_queue timing issues in this PR (discussed in https://github.com/nodejs/node/pull/29866#issuecomment-538841621)
• Re-flagging --experimental-json-modules (https://github.com/nodejs/node/pull/29754)
• Package-relative loading (https://github.com/nodejs/node/pull/29327)
• Unflagging --experimental-exports (https://github.com/nodejs/node/pull/29867)
• Bootstrap refactoring (https://github.com/nodejs/node/pull/29937)
• Resolution on exports fallback to main (https://github.com/nodejs/node/issues/29932)
• Conditional exports considerations (https://github.com/nodejs/node/pull/29978)
• Matching ESM extensions order (https://github.com/nodejs/node/pull/29974)

In addition to:

• Ensuring documentation has been fully reviewed for an unflagged modules release
• Sign off from the modules group.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines