test: add coverage for ESM external stack trace column numbers with prepareStackTrace

[Copilot]

Adds test coverage for PR #21562, which fixes incorrect column numbers in stack traces when using sourcemapInterceptor: 'prepareStackTrace' with ESM external modules.

Changes

• Test fixture: New ESM external package with single-line code containing padding and nested functions to test column accuracy
• Test case: Verifies stack trace columns are correct (>60), would fail if 62-char offset incorrectly applied (columns would be ~11 and ~52)
• Configuration: Added fixture to SSR externals config and .prettierignore to preserve single-line format

The test imports a module that throws through the external:

// esm-external-column-test/index.mjs (single line)
var _padding_______________________; export function outer(fn) { return inner(fn); } function inner(fn) { return fn(); }

// test verifies column numbers in stack:
//   at inner (file://.../index.mjs:1:114)  ✓ correct
//   at outer (file://.../index.mjs:1:73)   ✓ correct
// vs with bug:
//   at inner (file://.../index.mjs:1:52)   ✗ wrong (114-62)
//   at outer (file://.../index.mjs:1:11)   ✗ wrong (73-62)

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