feat: add preprocessor

[northwang-lucky]

Based your reply in the pull request (#96). Implementation for preprocessor.

[northwang-lucky]

@mrcrowl There are something new in this PR! Thank you for your great ideas. If possible, please review the code I submitted again!

• [x] Allow for more than one preprocessor.
• [x] The preprocessor fns take simple string input/output rather than TextDocument.
• [x] Delaying the preprocessing until just before the less.render(...) call.

[mrcrowl]

Now released as v2.0.0 in VS Code Marketplace.

[mrcrowl]

@northwang-lucky, can you confirm whether the released v2.0.0 version successfully supported your use case for adding a preprocessing step?

[northwang-lucky]

@mrcrowl A little busy lately, maybe I can try it over the weekend.