Fix @ on AZERTY on wasm

[Vrixyz]

Follow up on #149 ; I'm building on wasm and realized my fix is not sufficient, I can't input @ on wasm :'(.

In #149 we did a minimal change by checking only windows, but I think we're overly conservative, I'd like to discuss why this condition is needed, and make proper tests about it to validate its usefulness.

[Vrixyz]

support for ctrl-A was mentionned, but I can attest this PR does not break it. (it's ctrl-Q on my keyboard though, but same behaviour as current live web showcase)

[alice-i-cecile]

What about ALT behavior? Shouldn't those keys also be skipped, like in the original implementation?

[Vrixyz]

I think we actually don't want to prevent ALT, this PR enables me to input weird characters with altcodes (alt + keys): like the alt + (1;3;0) inputs é (https://www.alt-codes.net/)

[vladbat00]

I'm afraid this breaks the Cmd+A functionality on MacOS: selecting the text works, but it also inserts the "A" character afterwards.

[Vrixyz]

I'm afraid this breaks the Cmd+A functionality on MacOS: selecting the text works, but it also inserts the "A" character afterwards.

Indeed, I think a better fix would be to check for the user agent on web ; and keep this "ctrl+alt" special case gated for windows only.

An implementation of checking for user agent is available there: https://github.com/mvlabat/bevy_egui/pull/178/files#diff-420f4e08a25299e0e1fb09f4757e1d5d027a2278f37030c09ab9c06786cfa52eR85-R99

[Vrixyz]

I'm closing this and will work on previous comment