Update Conifer\Twig\FormHelper filter names to better match Form API method names

[sdunham]

Expected behavior

Conifer's twig form filter functions should be a closer match to the corresponding class methods they call. And possibly be prefixed with something like form_ to avoid confusion and possible collisions with future custom twig filters. Docs should also reflect these updated filter names where appropriate.

Actual behavior

They aren't!

Steps to reproduce behavior

N/A

What version of WordPress/PHP/Timber/Conifer are you using?

Latest Conifer version & docs

How did you install Conifer? (for example, via git clone, GitHub zip download, Composer/Packagist?)

N/A