fix(postcss): keep comment indentation stable

[Anber]

Motivation

stylelint --fix is not idempotent for comments inside indented Linaria templates: it adds the template's base indentation before trailing comments and removes it from continuation lines inside multi-line comments on every run.

Closes #1502.

Summary

• restore base indentation before a root node only when the whitespace actually starts the line
• preserve source indentation inside unchanged multi-line comment text
• cover both cases across three parse/stringify passes

Validation

• 5 Jest suites, 97 tests, 13 snapshots
• TypeScript typecheck
• ESLint
• Prettier

[changeset-bot]

🦋 Changeset detected

Latest commit: 8cc33ce

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

This PR includes changesets to release 9 packages

Name	Type
@linaria/postcss-linaria	Patch
@linaria/atomic	Patch
@linaria/babel-plugin-interop	Patch
@linaria/core	Patch
@linaria/react	Patch
@linaria/server	Patch
@linaria/stylelint	Patch
@linaria/stylelint-config-standard-linaria	Patch
linaria	Patch

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