differentiate metavariables and literals in target syntax

In the description of target syntax for `cabal v2-build`, it's not super clear which bits of the syntax are variables that the user should replace with their own text, and which are literal strings that should be copied verbatim into a target string. I *think* it's this way:

* variables: `package`, `ctype`, `ctypes`, `component`
* literals: `all`, `lib`, `libs`, `libraries`, `flib`, `flibs`, `foreign-libraries`, `exe`, `exes`, `executables`, `test`, `tests`, `bench`, `benches`, `benchmarks`, `:`

Some text describing which is which would help a lot; but I think even better would be distinguishing them typographically, e.g. by making variables italic or similar.

Start with the documentation describing target syntax for `cabal v2-build` and review the listed variables and literal strings. Done means the documentation clearly identifies which terms users replace and which they copy verbatim, preferably using distinct typography.

Written by the indexing model from the issue text.