Add decoration to focused item

[asger-semmle]

When following a link from the result viewer, we highlighted the target range simply by selecting it.

When this range is an identifier, it triggers vscode's occurrence-match highlighting, which makes it hard to see at a glance what was highlighted. This PR introduces decorators to highlight it a bit more, using the same colors as in search/replace.

The decoration is cleared on any selection change event in that editor.

Before

After

[jcreedcmu]

Code lgtm, but I still find the simultaneous decoration and selection of code to be a bit visually jarring in the case of multi-line matches. This is since, for some reason I still have not completely understood, selection includes one character's worth of whitespace at every newline, but decorations with isWholeLine: false do not, resulting in things like the following screenshot:

Do you suppose we could just rely on decoration only, and skip the selection-setting on https://github.com/github/vscode-codeql/pull/164/files#diff-73bb24db58f098e75356f2e05edb5e62R432 ?

[asgerf]

That's a good point.

I've experimented quite a bit with where to place the cursor, and found that for small ranges, bracket-highlighting is more attention-grabbing than our decorator which is why I didn't like placing the cursor at the beginning. But multi-line selections don't have that problem (as they're large) so I decided to just treat them differently.

WDYT?

[jcreedcmu]

Yeah, I think treating single-line and multi-line differently as you do is appropriate.