Support multi-word backticks like `some code` in RDoc

[eregon]

From ruby/timeout#92 (comment)

With RDoc supporting more of Markdown, many will expect that

Here is `some code`

works.

But unfortunately it doesn't for e.g. rescue Exception:

(from https://docs.ruby-lang.org/en/master/Timeout.html#method-c-timeout)

Somewhat related: #1230

[tompng]

One blocker of implementing this is the parsing mechanism that uses regexp and gsub.
We first need to remove that mechanism and improve rdoc's inline format parser (#1559)