Reduce gem package size

[pocke]

This pull request will reduce the gem package size by removing some files from the package.

I think removing yarn.lock will mostly reduce the size, because yarn.lock is 9.9KB.

I've confirmed this change reduced about 10KB, it is 20%.

# before this pull request
$ gem build language_server-protocol.gemspec
$ wc -c language_server-protocol-3.14.0.1.gem # it prints byte size.
45568 language_server-protocol-3.14.0.1.gem

# after this pull request
$ gem build language_server-protocol.gemspec
$ wc -c language_server-protocol-3.14.0.1.gem
35840 language_server-protocol-3.14.0.1.gem

But, I understand it will be a small impact in the real world. We have a high-speed network, so the 10KB difference can be ignored.
So if you think this change is not reasonable, feel free to close it.

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By the way, this change is based on rubocop's gemspec.
https://github.com/rubocop-hq/rubocop/blob/ebae59be1d9e05bec311ce63b62b182f7a5cd118/rubocop.gemspec#L19-L20

[mtsmfm]

@pocke Thanks!

[mtsmfm]

https://rubygems.org/gems/language_server-protocol

It's much smaller than before ❤️