Show Seq App settings that specify `Syntax` in fixed-width font

[nblumhardt]

Many app settings contain values that are URLs, templates, and text in other DSLs/programming languages.

In 2021.4, settings marked with [SeqAppSetting(..., Syntax="some non-whitespace-value")] will be rendered in fixed-width font.

For example, the new Seq.App.HttpRequest app's URL setting:

It's our hope that in later releases, settings will support additional syntax highlighting and completion options.