Trim down IPC json size

[edwin0cheng]

This PR try to trim down the json of proc macro IPC by ignoring token id if it equals to TokenId::unspecifed.

Test by following commands:

$ git clone https://github.com/gluon-lang/lsp-types.git  
$ cd lsp-types
$ export RA_LOG="proc_macro_api=debug"
$ rust-analyzer -q analysis-stats --load-output-dirs --with-proc-macro . 2> debug.log
$ cat debug.log | awk '/^\[DEBUG proc_macro_api::msg\] >/ {print substr($0,31)}' >expand.log
$ stat -c "%s" expand.log

Before: 37576726
After: 28551718

So it trimed down 25%.

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[lnicola]

changelog fix