add environment variable form of new CLI args, update docs

docs/release-notes.md

@@ -20,7 +20,6 @@
 * Added `data-*` attributes to the log parser page for external tools.
 * Fixed JSON validator showing incorrect error for update keys without a subkey.
 
-
 ### For mod authors
 This is a big release that includes the new APIs planned for SMAPI 4.0.0, alongside the old ones.
 
@@ -40,6 +39,7 @@ the C# mod that loads them is updated.
     _This adds support for many previously unsupported cases: proxied interfaces in return values or input arguments, proxied enums if their values match, generic methods, and more. Existing mod APIs should work fine as-is._
   * Mod files loaded through SMAPI APIs (including `helper.Content.Load`) are now case-insensitive, even on Linux.
 * Other improvements:
+  * Added [command-line arguments](technical/smapi.md#command-line-arguments) to toggle developer mode (thanks to Tondorian!).
   * Added `IContentPack.ModContent` property.
   * Added `Constants.ContentPath`.
   * Added `IAssetName` fields to the info received by `IAssetEditor` and `IAssetLoader` methods.  

docs/technical/smapi.md

@@ -33,11 +33,12 @@ argument | purpose
 `--uninstall` | Preselects the uninstall action, skipping the prompt asking what the user wants to do.
 `--game-path "path"` | Specifies the full path to the folder containing the Stardew Valley executable, skipping automatic detection and any prompt to choose a path. If the path is not valid, the installer displays an error.
 
-SMAPI itself recognises two arguments **on Windows only**, but these are intended for internal use
+SMAPI itself recognises five arguments **on Windows only**, but these are intended for internal use
 or testing and may change without warning. On Linux/macOS, see _environment variables_ below.
 
 argument | purpose
 -------- | -------
+`--developer-mode`<br />`--developer-mode-off` | Enable or disable features intended for mod developers. Currently this only makes `TRACE`-level messages appear in the console.
 `--no-terminal` | The SMAPI launcher won't try to open a terminal window, and SMAPI won't log anything to the console. (Messages will still be written to the log file.)
 `--use-current-shell` | The SMAPI launcher won't try to open a terminal window, but SMAPI will still log to the console. (Messages will still be written to the log file.)
 `--mods-path` | The path to search for mods, if not the standard `Mods` folder. This can be a path relative to the game folder (like `--mods-path "Mods (test)"`) or an absolute path.
@@ -49,6 +50,7 @@ can set temporary environment variables instead. For example:
 
 environment variable | purpose
 -------------------- | -------
+`SMAPI_DEVELOPER_MODE` | Equivalent to `--developer-mode` and `--developer-mode-off` above. The value must be `true` or `false`.
 `SMAPI_MODS_PATH` | Equivalent to `--mods-path` above.
 `SMAPI_NO_TERMINAL` | Equivalent to `--no-terminal` above.
 `SMAPI_USE_CURRENT_SHELL` | Equivalent to `--use-current-shell` above.

src/SMAPI/Program.cs

@@ -200,6 +200,12 @@ namespace StardewModdingAPI
                 // get from environment variables
                 if (string.IsNullOrWhiteSpace(rawModsPath))
                     rawModsPath = Environment.GetEnvironmentVariable("SMAPI_MODS_PATH");
+                if (developerMode is null)
+                {
+                    string rawDeveloperMode = Environment.GetEnvironmentVariable("SMAPI_DEVELOPER_MODE");
+                    if (rawDeveloperMode != null)
+                        developerMode = bool.Parse(rawDeveloperMode);
+                }
 
                 // normalize
                 modsPath = !string.IsNullOrWhiteSpace(rawModsPath)