Add si locale for Sinhala language

[sujithranga]

Thanks for your interest in plotly.js!

Translations:

• Please @ mention a few other speakers of this language who can help review your translations.
  @wnilanka @dinithija
• If you've omitted any keys from dist/translation_keys.txt - which means they will fall back on the US English text - just make a short comment about why in the PR description: the English text works fine in your language, or you would like someone else to help translating those, or whatever the reason.
  // I need to check further before translate several strings but merge this PR
• You should only update files in lib/locales/, not those in dist/

Features, Bug fixes, and others:

Before opening a pull request, developer should:

• provide a title and write an overview of what the PR attempts to do with a link to the issue they are trying to address.
  Added si locale
• select the Allow edits from maintainers option (see this article for more details).

After opening a pull request, developer:

• should create a new small markdown log file using the PR number e.g. 1010_fix.md or 1010_add.md inside draftlogs folder as described in this README, commit it and push.

[archmoj]

Thanks for the PR.
Would you please write the title in English?

[archmoj]

Using non-ASCII characters in the branch name as well as commit messages may cause issues for certain developers.
I suggest you kindly open a new PR with those adjustments.
Thank you!

[sujithranga]

Would you please write the title in English?

Using non-ASCII characters in the branch name as well as commit messages may cause issues for certain developers. I suggest you kindly open a new PR with those adjustments. Thank you!

Anyway, renamed the title and rest of things are not considerable problem :)

[wnilanka]

No incorrect strings

[dinithija]

good

[archmoj]

• should create a new small markdown log file using the PR number e.g. 1010_fix.md or 1010_add.md inside draftlogs folder as described in this README, commit it and push.

Above task is not completed. Would you mind creating draftlogs/6238_add.md file please?

[alexcjohnson]

💃 Thanks @sujithranga - and thank you @wnilanka and @dinithija for reviewing 🙇