feat: support Lake for building Lean core oleans (#3886)

This is from a ~~pair~~triple programming session with @tydeu and
@mhuisi.

If stage 1 is built with `-DUSE_LAKE=ON`, the CMake run will generate
`lakefile.toml` files for the root, `src`, and `tests`. These Lake
configuration files can then be used to build core oleans. While they do
not yet allow Lake to be used to build the Lean binaries. they do allow
Lake to be used for working interactively with the Lean source. In our
preliminary experiments, this allowed updates to `Init.Data.Nat` to be
noticed automatically when reloading downstream files, rather than
requiring a full manual compiler rebuild. This will make it easier to
work on the system.

As part of this change, Lake is added to stage 0. This allows Lake to
function in `src`, which uses the stage 0 toolchain.

---------

Co-authored-by: Mac Malone <tydeu@hatpress.net>
Co-authored-by: Sebastian Ullrich <sebasti@nullri.ch>

script/lib/update-stage0

@@ -15,4 +15,19 @@ for f in $(git ls-files src ':!:src/lake/*' ':!:src/Leanc.lean'); do
         cp $f stage0/$f
     fi
 done
+
+# special handling for Lake files due to its nested directory
+# copy the README to ensure the `stage0/src/lake` directory is comitted
+for f in $(git ls-files 'src/lake/Lake/*' src/lake/Lake.lean src/lake/README.md ':!:src/lakefile.toml'); do
+    if [[ $f == *.lean ]]; then
+        f=${f#src/lake}
+        f=${f%.lean}.c
+        mkdir -p $(dirname stage0/stdlib/$f)
+        cp ${CP_C_PARAMS:-} $CSRCS/$f stage0/stdlib/$f
+    else
+        mkdir -p $(dirname stage0/$f)
+        cp $f stage0/$f
+    fi
+done
+
 git add stage0