Improve severity labels in Actions logs and tweak colors (#36993)

Add support for error, warning, notice, and debug log commands with bold
label prefixes and colored backgrounds matching GitHub's style. Parse
both `##[cmd]` and `::cmd args::` formats.

Also improved the severity colors globally and added a devtest page for
these.

---------

Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>

web_src/css/modules/message.css

@@ -41,9 +41,9 @@
   margin-bottom: 1em;
 }
 
-.ui.info.message .header,
-.ui.blue.message .header {
-  color: var(--color-blue);
+.ui.message .header {
+  color: inherit;
+  filter: saturate(2);
 }
 
 .ui.info.message,
@@ -55,12 +55,6 @@
   border-color: var(--color-info-border);
 }
 
-.ui.success.message .header,
-.ui.positive.message .header,
-.ui.green.message .header {
-  color: var(--color-green);
-}
-
 .ui.success.message,
 .ui.attached.success.message,
 .ui.positive.message,
@@ -70,12 +64,6 @@
   border-color: var(--color-success-border);
 }
 
-.ui.error.message .header,
-.ui.negative.message .header,
-.ui.red.message .header {
-  color: var(--color-red);
-}
-
 .ui.error.message,
 .ui.attached.error.message,
 .ui.red.message,
@@ -87,11 +75,6 @@
   border-color: var(--color-error-border);
 }
 
-.ui.warning.message .header,
-.ui.yellow.message .header {
-  color: var(--color-yellow);
-}
-
 .ui.warning.message,
 .ui.attached.warning.message,
 .ui.yellow.message,