Missing support for ms932, koi8-ru encoding labels

[Ms2ger]

Blocks #5601.

• Spec: https://encoding.spec.whatwg.org/#names-and-labels.
• Test: /encoding/textdecoder-labels.html.

This was fixed in lifthrasiir/rust-encoding@40717f5 for v0.3 (which is unfinished). Not sure if it's worth fixing in the 0.2 branch or if we should just wait for 0.3 to be finished.

CC @SimonSapin @lifthrasiir

[nox]

@SimonSapin Are we supposed to switch to encoding_rs?

[SimonSapin]

I want to do that eventually, yes. I just hasn’t been pressing so far.

[dralley]

Since it's 4 years later and the encoding_rs switched happened quite a while ago, can this be closed?

[jdm]

We now pass /encoding/textdecoder-labels.html.