Added log custom command & exported log levels (#9055)

# Description
- Log levels are now exported members of `std log`, e.g. `std log
CRITICAL_LEVEL`
- Added `std log custom` command that allows to declare custom message
format (actual message replaces `%MSG%` in the template) with
user-defined log level.


# User-Facing Changes
New possibilities included in description

# Tests + Formatting
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crates/nu-std/tests/test_logger.nu

@@ -36,3 +36,41 @@ export def test_debug [] {
     assert no message INFO debug 
     assert message DEBUG debug DBG
 }
+
+
+def "run custom" [system_level, format, message_level] {
+    do {
+        ^$nu.current-exe -c $'use std; NU_LOG_LEVEL=($system_level) std log custom "test message" "($format)" ($message_level)' 
+    } | complete | get -i stderr
+}
+
+def "assert custom message" [system_level, format, message_level] {
+    let output = (run custom $system_level $format $message_level)
+    assert equal ($output | str trim -r) ($format | str replace "%MSG%" "test message")
+}
+
+def "assert custom message contains" [system_level, format, message_level, tested_str] {
+    let output = (run custom $system_level $format $message_level)
+    assert ($output | str contains  $tested_str)
+    assert ($output | str contains "test message")
+}
+
+def "assert custom message not contains" [system_level, format, message_level, tested_str] {
+    let output = (run custom $system_level $format $message_level)
+    assert (not ($output | str contains  $tested_str))
+    assert ($output | str contains "test message")
+}
+
+def "assert no custom message" [system_level, format, message_level] {
+    let output = (run custom $system_level $format $message_level)
+    assert equal ($output | str trim -r) ""
+}
+
+export def test_custom [] {
+    assert no custom message (log ERROR_LEVEL) "%MSG%" (log DEBUG_LEVEL)
+    assert custom message (log DEBUG_LEVEL) "%MSG%" (log INFO_LEVEL)
+    assert custom message (log WARNING_LEVEL) $"my_msg: %MSG%" (log CRITICAL_LEVEL)
+
+    assert custom message contains (log DEBUG_LEVEL) $"(ansi yellow)[%LEVEL%]MY MESSAGE: %MSG% [%DATE%](ansi reset)" (log WARNING_LEVEL) (log WARNING_LEVEL_PREFIX)
+    assert custom message not contains (log DEBUG_LEVEL) $"(ansi yellow)MY MESSAGE: %MSG% [%DATE%](ansi reset)" (log WARNING_LEVEL) (log WARNING_LEVEL_PREFIX)
+}