Notes: store line numbers on save

doc/line_numbers.md

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+# Line Numbers
+
+In some sense, line numbers aren't meaningful in Synless:
+
+- For screen readers, there is literally no such thing as a line.
+- The notation you display with may be very different from the notation printed. E.g. you could have
+  Rust displayed with indentation instead of braces, and Python displayed with braces instead of
+  indentation.
+
+However, they are important when interfacing with the outside world. Thus we'll store them exactly
+when doing so! Upon save, mark every node in the document with the _first_ line number it was
+printed on. Display the line number for the current node in the status bar. You can jump to a node
+by line.