fix(postcss-linaria): enable CI tests

[Anber]

Motivation

@linaria/postcss-linaria has a Jest test suite, but it hasn’t been exercised by CI because the package had no test script (see #1455).

When enabling it, a few tests fail on a clean checkout because:

• stylelint resolves extends relative to CWD unless configBasedir is provided.
• placeholder identifiers were using pcss_lin (underscore), which doesn’t match kebab-case rules and doesn’t match the config’s ignoreComments: [/pcss-lin/].
• location offsets needed different adjustment for source.start vs source.end.

Summary

• Add test script for @linaria/postcss-linaria so turbo run test/check:all can execute it.
• Make @linaria/postcss-linaria#test depend on @linaria/postcss-linaria#build (stylelint config uses the published entry, which requires built output).
• Fix locationCorrection offset math for end positions.
• Switch placeholder prefix to pcss-lin (kebab-case) to match stylelint rules and ignore patterns.
• Fix stylelint tests by passing configBasedir.

Test plan

• pnpm turbo run test --filter @linaria/postcss-linaria
• pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 4c46401

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@linaria/postcss-linaria	Patch
@linaria/stylelint-config-standard-linaria	Patch

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