fix(css): show filename in CSS minification warnings for .css?inline

[Jah-yee]

Good day

When CSS minification produces warnings, the filename was showing as <stdin> which made it difficult to track down which file had the issue.

This change passes the actual filename to esbuild's sourcefile option and to lightningcss, so that warning messages now contain the correct filename instead of <stdin>.

#15915

Changes

• Add filename parameter to minifyCSS function with default value defaultCssBundleName
• Pass filename to esbuild's sourcefile option for better warning messages
• Pass actual filename to lightningcss instead of hardcoded defaultCssBundleName
• For inlined CSS, use module ID as the filename

Before

warnings when minifying css:
▲ [WARNING] Unexpected "{" [css-syntax-error]

    <stdin>:484:1:
      484 │  {
          ╵  ^

After

The warning message will now show the actual filename instead of <stdin>.

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Thank you for your attention. If there are any issues or suggestions, please leave a comment and I will address them promptly.

Warmly,
Jah-yee

I've removed the marked fix for #15915 for now as the issue has linked PRs that are different to this that we may still want to address.