Fix identifier highlight in injected languages.

[Earthcomputer]

Identifier highlighting was disabled for single-line editors, which accounts for injected languages inside single-line Java string literals. According to git blame, the original intention of this was to disable identifier highlighting in dialog editors, so I switched the check to explicitly check that.

My use case is that I'm developing a plugin which makes use of a MultiHostInjector for a DSL spread across multiple Java string literals. It would significantly improve UX if identifier highlighting was possible in this use case. I could not find any clean workaround which doesn't involve modifying IntelliJ in this way. Besides, I consider this a bug and it's nice to get it fixed :)

Before the fix:

After the fix:

[BasLeijdekkers]

Hi @Earthcomputer , Thank you for your contribution. Could you create a YouTrack issue for this as well? That way it becomes part of the release notes and such.

[Earthcomputer]

Hi, sorry for the delay, I've been away. I can take a look this evening.

[Earthcomputer]

I addressed the review, and made a YouTrack issue: https://youtrack.jetbrains.com/issue/IDEA-346533/Identifier-highlighting-doesnt-work-in-single-line-language-injections

[BasLeijdekkers]

Hello, sorry for the delay. Could you squash the two commits into one? That makes it easier to apply the changes to our internal repository.

[Earthcomputer]

@BasLeijdekkers done