Update plotting code for ggplot2 deprecations

[foertsch]

Fixes #27

This PR updates plot_density_() to avoid current ggplot2 deprecation warnings.

Summary

• replace aes_string() with tidy-eval aes(...) using .data
• replace element_line(size = ...) with element_line(linewidth = ...)
• update the roxygen import accordingly

Why

While checking plot_density() on current master, the plotting path worked correctly but emitted ggplot2 deprecation warnings for:

• aes_string()
• element_line(size = ...)

This patch keeps behavior the same while making the plotting code compatible with current ggplot2 expectations.

Verification

Tested locally with:

library(Nebulosa)
data <- SeuratObject::pbmc_small
p <- plot_density(data, "CD3E")
inherits(p, "ggplot")

and:

testthat::test_file("tests/testthat/test-plot_density.R")

The direct call still returns a ggplot, and test-plot_density.R now passes without warnings.

Note: I originally looked at issue #22, but current master already appears to handle that Seurat-related failure. While verifying it, I noticed these ggplot2 deprecation warnings and prepared this small cleanup PR instead.