cli: ability to disable default configs

My motivation is mostly to address #5098 , but in a lot of
software there's a flag to disable sourcing default configs,
which is useful in a lot contexts.

A more granular (or just atlernative/better) approaches
would require more design and debate, while this seems
simple and immediately useful (at least to some).

cli/src/config.rs

@@ -488,13 +488,17 @@ pub fn default_config_layers() -> Vec<ConfigLayer> {
     // Syntax error in default config isn't a user error. That's why defaults are
     // loaded by separate builder.
     let parse = |text: &'static str| ConfigLayer::parse(ConfigSource::Default, text).unwrap();
-    let mut layers = vec![
-        parse(include_str!("config/colors.toml")),
-        parse(include_str!("config/merge_tools.toml")),
-        parse(include_str!("config/misc.toml")),
-        parse(include_str!("config/revsets.toml")),
-        parse(include_str!("config/templates.toml")),
-    ];
+    let mut layers = if let Some(_) = env::var("JJ_NO_DEFAULT_CONFIG") {
+        vec![]
+    } else {
+        vec![
+            parse(include_str!("config/colors.toml")),
+            parse(include_str!("config/merge_tools.toml")),
+            parse(include_str!("config/misc.toml")),
+            parse(include_str!("config/revsets.toml")),
+            parse(include_str!("config/templates.toml")),
+        ]
+    };
     if cfg!(unix) {
         layers.push(parse(include_str!("config/unix.toml")));
     }