Figure out what to do about new JavaScript private member #syntax

[jattasNI]

The JavaScript feature to set class members as private by prefixing their name with # is now supported in all modern browsers. This issue tracks its impact on our coding conventions.

Things to look into:

1. do our conventions currently allow the syntax in JS code? Should they?
2. do our conventions currently allow the syntax in TS code? Should they?
   • TypeScript obviously has its own private member syntax (via the private keyword). We should see if they've expressed an opinion about how that syntax will interact with the JS syntax.

[jattasNI]

TypeScript's documentation for member visibility states that both the private syntax and the # are supported (if you target ES2021 or less, it uses WeakMap rather than the native syntax for #). It doesn't take a position on using one or the other, besides their differences in behaviour (edit-time vs run-time privacy).

[jattasNI]

Some thoughts from today's meeting:

• should allow the # in our conventions but not take a stand on using it vs private until NI devs and the community have time to form opinions about the new syntax
• shouldn't use the # syntax in TS codebases that target ES2021 or earlier because the WeakMap approach is likely poor performing