feat: adding background using markupModel.addLineHighligher()

[synopss]

Fixes : #14

Notes :

• this probably can be improved, this is more a poc than anything else.
• using TextAttributes is seen as a deprecated way to do things, TextAttributesKey should be used instead. I didn't use it to keep things close to the existing code base.

[chylex]

Is it okay if I push changes directly into your branch? You made the PR from the main branch so it's a little awkward when the main branch is tied up to a PR, but if you're not going to do anything else with the branch it should be fine.

[synopss]

Fine for me.

[chylex]

Found a few issues:

• No support for light themes which makes the line unreadable
• There is no conflict resolution for multiple inspections with different severities on the same line
• It seems to override font style and draws the whole line in italics
• There are some unnecessary allocations

As you said, it's a PoC so it's good to have as a jumping off point, but I cannot merge it without fixing all of these issues.

I'll fix some of these so that I can start trying it out. I think background colors could help, but I want to run it across a few people before I commit to having the feature at all, or maybe having it as an option; if neither, it could always be implemented as a separate addon plugin (or a completely independent plugin).