fix: handle lakefile.toml

[gaetanserre]

Description

Change the lakeFile.fileName check in LeanContext.lean/initializeLakeContext in order to handle .toml lakefile, introduced in Lean v4.8.0.

[mhuisi]

This repository is unfortunately effectively unmaintained, but the change looks simple enough and I've quickly checked that lake setup-file works as expected with the TOML config as well, so this seems fine to merge.