Use the start.lua file in data/core as a template

Instead of having a separate start.lua.in file in the scripts directory
and no start.lua file in data/core we use the file data/core/start.lua
as a template for Meson to generate the final start.lua file for release.

In this way people naturally trying to run lite-xl from the source folder
will have a start.lua file albeit without a resolved version number.

Otherwise, when using run-local script or the meson install command the
meson-generated start.lua file will be used as it should be.

scripts/run-local

@@ -76,10 +76,13 @@ copy_lite_build () {
     cp "$builddir/src/lite-xl" "$bindir"
   fi
   mkdir -p "$datadir/core"
-  cp "$builddir/start.lua" "$datadir/core"
   for module_name in core plugins colors fonts; do
     cp -r "data/$module_name" "$datadir"
   done
+  # The start.lua file is generated by meson in $builddir but
+  # there is already a start.lua file in data/core so the command below
+  # should be executed after we copy the data/core directory.
+  cp "$builddir/start.lua" "$datadir/core"
 }
 
 run_lite () {