Add field / column util for explicitly treating an expression as a field

[anthonynorth]

Accessors interpret non-symbol expressions as values, not columns. The escape hatch is to convert the expression to a name, which isn't obvious.

Related #54

[anthonynorth]

An alias for rlang::sym() or as.name() adds little value. The lack of clarity was due to poor documentation for tidy-eval in accessor and scale parameter types

https://anthonynorth.github.io/rdeck/reference/accessor.html https://anthonynorth.github.io/rdeck/reference/scale.html