Fix parsing extra heartbeats: invalid character 'U' in string escape code

[alanhamlett]

Logs:

{"caller":"cmd/params/params.go:401","func":"params.LoadHeartbeatParams","level":"error","message":"failed to read extra heartbeats: failed parsing: failed to json decode from data \"[{\\\"entity\\\": \\\"C:\\\\Users\\\\pikac\\\\AppData\\\\Local\\\\nvim\\\\lua\\\\plugins\\\\termdebug.lua\\\", \\\"timestamp\\\": 1676881606, \\\"is_write\\\": true, \\\"alternate_language\\\": \\\"lua\\\"}]\\n\": invalid character 'U' in string escape code","now":"2023-02-20T18:56:46+10:30","os/arch":"windows/amd64","version":"v1.66.0"}

Related to wakatime/wakatime-cli#797, but might be a bug in vim-wakatime's JSON rendering.

Originally reported in #156.

[alanhamlett]

It might be fixed with a4c66fa. @xanthus58 can you check?

Edit: Reverted a4c66fa because it broke the plugin