fmt: fix write_char not respecting indent string

take default indent string from env!

src/fmt.rs

@@ -2,11 +2,19 @@
 
 use std::fmt::{Display, Write};
 
+/// The default indentation string. Defaults to four spaces.
+const INDENTATION: &str = {
+    match option_env!("DOUGHLANG_INDENT") {
+        Some(indent) => indent,
+        None => "    ",
+    }
+};
+
 impl<W: Write + ?Sized> FmtAdapter for W {}
 pub trait FmtAdapter: Write {
     /// Indents by one level.
     fn indent(&mut self) -> Indent<'_, Self> {
-        Indent::new(self, "    ")
+        Indent::new(self, INDENTATION)
     }
 
     /// Pastes `indent` after each newline.
@@ -99,7 +107,7 @@ impl<F: Write + ?Sized> Write for Indent<'_, F> {
     }
     fn write_char(&mut self, c: char) -> std::fmt::Result {
         if self.needs_indent {
-            self.f.write_str("    ")?;
+            self.f.write_str(self.indent)?;
         }
         self.needs_indent = c == '\n';
         self.f.write_char(c)