Add new proofreader

What does this MR do?

https://docs.gitlab.com/ee/development/i18n/proofreader.html#become-a-proofreader

Language: Sinhalese/Sinhala
Native name: සිංහල
Code: si (gitlab-wide)

Note: crowdin use language code as si_LK, it's their mistake and do not use it. they can't fix it now due that may break their integrations etc.

If you request to become the first proofreader for a language and there are no GitLab or Core team members who speak that language, we request links to previous translation work in other communities or projects.

https://www.transifex.com/user/profile/HelaBasa/
https://crowdin.com/profile/HelaBasa