Logger constants refactored, format argument added, better formatting of failed (non) equality assertions (#9315)

# Description
I have (hopefully) simplified the `log.nu` internal structure and added
customizable log format for all `log` commands

# User-Facing Changes
- [x] Replaced constants with env records for: 
    - ansi (newly added)
    - log level
    - prefix
    - short prefix
- [x] Added `format` argument to all log commands
- [x] Assertions for (not) equality (equal, not equal, greater,
lesser...) now put left and right values inside `'` quotes, so the
assertions for strings are more meaningful
- [x] Documented the %-formatting of log messages

# Tests + Formatting
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---------

Co-authored-by: amtoine <stevan.antoine@gmail.com>

crates/nu-std/tests/logger_tests/test_log_custom.nu

@@ -0,0 +1,41 @@
+use std *
+use commons.nu *
+
+def run-command [
+    system_level: string,
+    message: string,
+    format: string,
+    log_level: int,
+    --level-prefix: string,
+    --ansi: string
+] {
+    do {
+        if ($level_prefix | is-empty) {
+            if ($ansi | is-empty) {
+                ^$nu.current-exe --commands $'use std; NU_LOG_LEVEL=($system_level) std log custom "($message)" "($format)" ($log_level)'
+            } else {
+                ^$nu.current-exe --commands $'use std; NU_LOG_LEVEL=($system_level) std log custom "($message)" "($format)" ($log_level) --ansi "($ansi)"'
+            }
+        } else {
+            ^$nu.current-exe --commands $'use std; NU_LOG_LEVEL=($system_level) std log custom "($message)" "($format)" ($log_level) --level-prefix "($level_prefix)" --ansi "($ansi)"'
+        }
+    }  | complete | get --ignore-errors stderr
+}
+
+export def test_errors_during_deduction [] {
+    assert str contains (run-command "DEBUG" "msg" "%MSG%" 25) "Cannot deduce log level prefix for given log level"
+    assert str contains (run-command "DEBUG" "msg" "%MSG%" 25 --ansi (ansi red)) "Cannot deduce log level prefix for given log level"
+    assert str contains (run-command "DEBUG" "msg" "%MSG%" 25 --level-prefix "abc") "Cannot deduce ansi for given log level"
+}
+
+export def test_valid_calls [] {
+    assert equal (run-command "DEBUG" "msg" "%MSG%" 25 --level-prefix "abc" --ansi (ansi default) | str trim --right) "msg"
+    assert equal (run-command "DEBUG" "msg" "%LEVEL% %MSG%" 20 | str trim --right) $"($env.LOG_PREFIX.INFO) msg"
+    assert equal (run-command "DEBUG" "msg" "%LEVEL% %MSG%" --level-prefix "abc" 20 | str trim --right) "abc msg"
+    assert equal (run-command "INFO" "msg" "%ANSI_START%%LEVEL% %MSG%%ANSI_STOP%" $env.LOG_LEVEL.CRITICAL | str trim --right) $"($env.LOG_ANSI.CRITICAL)CRT msg(ansi reset)"
+}
+
+export def test_log_level_handling [] {
+    assert equal (run-command "DEBUG" "msg" "%LEVEL% %MSG%" 20 | str trim --right) $"($env.LOG_PREFIX.INFO) msg"
+    assert equal (run-command "WARNING" "msg" "%LEVEL% %MSG%" 20 | str trim --right) ""
+}