Set ft=markdown.agda for *.lagda.md

This allows the markdown treesitter plugin to run automatically.
Previously the intention was to set ft=agda.markdown, but this indicates
that agda is the primary filetype, when this is not true. Furthermore,
this would not be set since it would be overwritten by the *.lagda.*
autocommand.

Fixes #157

ftdetect/agda.vim

@@ -1,5 +1,5 @@
 au BufNewFile,BufRead *.agda setlocal filetype=agda
 au BufNewFile,BufRead *.lagda setlocal filetype=agda
-au BufNewFile,BufRead *.lagda.md setlocal filetype=agda.markdown
 au BufNewFile,BufRead *.lagda.* setlocal filetype=agda
+au BufNewFile,BufRead *.lagda.md setlocal filetype=markdown.agda