Expand formula interface to aesthetics, facets, labels, and encode

[Copilot]

Summary

Expands the formula shorthand ~ var to work in all aesthetic mappings and variable-name arguments throughout the package.

What changed

Core implementation (R/utils.R, R/gglite.R, R/facet.R, R/component.R):

• New as_var() utility converts a one-sided single-term formula ~ var to the character string "var"; all other values pass through unchanged
• g2(): named aesthetic arguments now accept ~ var formulas (e.g., color = ~ species); the positional-formula detection is fixed to only trigger for the unnamed first argument
• encode(): all arguments accept ~ var formulas
• facet_rect() / facet_circle(): variable arguments accept ~ var formulas
• labels_(): the text argument (and all options) accept ~ var formulas

Usage

# Before (still works):
g2(iris, x = 'Sepal.Width', y = 'Sepal.Length', color = 'Species') |>
  facet_rect(x = 'Species') |>
  labels_(text = 'Sepal.Length')

# After (preferred):
g2(iris, Sepal.Length ~ Sepal.Width, color = ~ Species) |>
  facet_rect(x = ~ Species) |>
  labels_(text = ~ Sepal.Length)

Documentation updates

• All @examples in R source files updated to use the formula interface
• All examples/*.Rmd files updated to use the formula interface
• vignettes/gglite.Rmd updated to lead with the formula interface and document the character string interface as an alternative in a dedicated subsection

Tests added

• g2() with all-named formula aesthetics (x = ~ mpg, y = ~ hp, color = ~ cyl)
• g2() with mixed positional formula + formula aesthetic (hp ~ mpg, color = ~ cyl)
• encode() with formula aesthetics
• facet_rect() and facet_circle() with formula variables
• labels_() with text = ~ var
• as_var() error on multi-term formula