Use indentation settings in dx fmt and vscode extension

Adds the ability to specify an indent string to public autofmt methods - either
a sequence of spaces or a tab character.

Get the indentation style and size from rustfmt for dx fmt, or from the editor
settings for the vscode extension.

Closes #1595

packages/autofmt/src/indent.rs

@@ -0,0 +1,108 @@
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum IndentType {
+    Spaces,
+    Tabs,
+}
+
+#[derive(Debug, Clone)]
+pub struct IndentOptions {
+    width: usize,
+    indent_string: String,
+}
+
+impl IndentOptions {
+    pub fn new(typ: IndentType, width: usize) -> Self {
+        assert_ne!(width, 0, "Cannot have an indent width of 0");
+        Self {
+            width,
+            indent_string: match typ {
+                IndentType::Tabs => "\t".into(),
+                IndentType::Spaces => " ".repeat(width),
+            },
+        }
+    }
+
+    /// Gets a string containing one indent worth of whitespace
+    pub fn indent_str(&self) -> &str {
+        &self.indent_string
+    }
+
+    /// Computes the line length in characters, counting tabs as the indent width.
+    pub fn line_length(&self, line: &str) -> usize {
+        line.chars()
+            .map(|ch| if ch == '\t' { self.width } else { 1 })
+            .sum()
+    }
+
+    /// Estimates how many times the line has been indented.
+    pub fn count_indents(&self, mut line: &str) -> usize {
+        let mut indent = 0;
+        while !line.is_empty() {
+            // Try to count tabs
+            let num_tabs = line.chars().take_while(|ch| *ch == '\t').count();
+            if num_tabs > 0 {
+                indent += num_tabs;
+                line = &line[num_tabs..];
+                continue;
+            }
+
+            // Try to count spaces
+            let num_spaces = line.chars().take_while(|ch| *ch == ' ').count();
+            if num_spaces >= self.width {
+                // Intentionally floor here to take only the amount of space that matches an indent
+                let num_space_indents = num_spaces / self.width;
+                indent += num_space_indents;
+                line = &line[num_space_indents * self.width..];
+                continue;
+            }
+
+            // Line starts with either non-indent characters or an unevent amount of spaces,
+            // so no more indent remains.
+            break;
+        }
+        indent
+    }
+}
+
+impl Default for IndentOptions {
+    fn default() -> Self {
+        Self::new(IndentType::Spaces, 4)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn count_indents() {
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 4).count_indents("no indentation here!"),
+            0
+        );
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 4).count_indents("    v += 2"),
+            1
+        );
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 4).count_indents("        v += 2"),
+            2
+        );
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 4).count_indents("          v += 2"),
+            2
+        );
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 4).count_indents("\t\tv += 2"),
+            2
+        );
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 4).count_indents("\t\t  v += 2"),
+            2
+        );
+        assert_eq!(
+            IndentOptions::new(IndentType::Spaces, 2).count_indents("    v += 2"),
+            2
+        );
+    }
+}