feat: include_str macro for static file inclusion

DocGen4/IncludeStr.lean

@@ -0,0 +1,21 @@
+import Lean
+
+namespace DocGen4
+
+open Lean System IO Lean.Elab.Term
+
+syntax (name := includeStr) "include_str" str : term
+
+@[termElab includeStr] def includeStrImpl : TermElab := λ stx expectedType? => do
+    let str := stx[1].isStrLit?.get!
+    let path := FilePath.mk str
+    if ←path.pathExists then
+      if ←path.isDir then
+        throwError s!"{str} is a directory"
+      else
+        let content ← FS.readFile path
+        return mkStrLit content
+    else
+      throwError s!"\"{str}\" does not exist as a file"
+
+end DocGen4