Summary

Tidy up DiscordEmbedBuilder. I felt it needed some housekeeping.

Changes proposed

Convert properties from statement body to expression body where possible (I believe they were this way before as a way to work around a VS bug. I don't know if this bug still happens)
Rename fields and method parameters from snake_case to camelCase (I believe this change was already bound to happen so I thought it to be a good fit for this PR)
Remove redundant else statements
Change redundant field-backed properties to auto-properties
Tidy up Build method - use Optional<T>.IfPresent instead of HasValue and Value
Remove unnecessary ReadOnlyCollection wrapper around Fields, as an implicit cast is enough. Note that this allows outside modification of the underlying list by casting, but I think that would be a specific enough use case that there's no point working around it

Is this any better in terms of effort than updating the readme?