fix: Restore lint icon functionality by adding a temporary dedicated gutter

[Orphaeus]

Added an additional gutter to display lint icons without removing line numbers. This gutter is created upon lint if any issues exist, and removed if not.

I found #169 while experiencing the issue it discusses (disappearing line numbers upon lint), and I see that it was decided in 643e6bc to simply disable the lint icon functionality. I also saw Nathan's comment here:

I am in favor of (. . .) generally not trying to modify the script editor because I expect that we will have regressions over time and that is going to bring some maintenance burden.

and don't wish to argue with that assessment! I am simply uncertain how likely this PR is to add burden down the road, and wanted to offer a fix if others think it is viable.

I believe the modifications I made should be less likely to break things, since, unlike the original implementation, they don't override any existing editor functionality. Having icons to help locate issues identified by the linter is a great QoL feature, and I'd like to help preserve it if I can.

Please check if the PR fulfills these requirements:

• The commit message follows our guidelines.
• For bug fixes and features:
  • You tested the changes.

Related issue (if applicable): #169

What kind of change does this PR introduce?
Small visual/behavioral adjustment to restore disabled functionality.

Does this PR introduce a breaking change?
Not to my knowledge.

New feature or change

What is the current behavior?
The version of the plugin found in the asset library (0.16.1) removes line numbers from files when the linter is run. The functionality that caused this (applying icons to the lines where issues are found) was disabled last month in 643e6bc.

What is the new behavior?
This restores the use of icons to mark linter issues, but does so in a way that does not override any existing editor functionality. If any issues are found when the linter is run, a new gutter is created at index 2 and used to display the icons. The next time the linter is run, if there are no issues, the gutter is removed.

Other information
When removing the gutter, every gutter is checked to see if it has the name "lint_icon_gutter", rather than assuming that the desired gutter is found at index LINT_ICON_GUTTER. This is to prevent conflicts in case other addons have also added gutters.