return `default` for .sky files in getBuildifierFileType (#267)
return `default` for .sky files in getBuildifierFileType (#267)

## What 
I noticed that the vscode extension's outputs for copybara configuration files are different from the CLI's output.
## Why
`getBuildifierFileType` is out of sync with <https://github.com/bazelbuild/buildtools/blob/a43aed7014c840a4c20c84958f3f15df5da780f5/build/lex.go#L125-L151>

## Ref
- https://github.com/bazelbuild/buildtools/pull/560
- [getBuildifierFileType](https://github.com/bazelbuild/vscode-bazel/blob/ffd1450a4222825db11533aac5f5c7aa51ef50eb/src/buildifier/buildifier.ts#L116-L152)
bazelbuild/vscode-bazel,bazelbuild/vscode-bazel