Use syntax injection for wikilink highlighting

[memeplex]

As described in #479 (comment):

• Simpler declarative implementation (I think you can get rid of the entire syntaxDecorations.ts file)
• Better performance
• No flickering and weird intermediate colorization (see screenshot below)
• Customizable color (using standard token highlighting settings)
• Color appropriately changes with theme (sensible default that uses base link color)

[memeplex]

You might be interested in primer/github-vscode-theme#320. I noticed that syntax injected wikilinks are not released yet and that wikilinks decorations are conflicting with such a popular theme may be a reason to push those changes.

[svsool]

It should be released now.