internal/js: Add default mainFields for CSS builds

Closes #14614

internal/js/esbuild/options.go

@@ -402,6 +402,10 @@ OUTER:
 		sourcesContent = api.SourcesContentExclude
 	}
 
+	if opts.IsCSS && opts.MainFields == nil {
+		opts.MainFields = []string{"style", "main"}
+	}
+
 	opts.compiled = api.BuildOptions{
 		Outfile:       outFile,
 		Bundle:        true,

tpl/css/build_integration_test.go

@@ -304,7 +304,7 @@ disableKinds = ["taxonomy", "term"]
 }
 -- layouts/home.html --
 {{ with resources.Get "css/main.css"  }}
-{{ with . | css.Build (dict "minify" true "mainFields" (slice "style"))  }}
+{{ with . | css.Build (dict "minify" true)  }}
 	 CSS size: {{ .Content | len }}|{{ .RelPermalink }}
 {{ end }}
 {{ end }}