input_code_editor(): fix caret drift caused by prism theme font-family override

[gadenbuie]

Closes #1303

Summary

Prism Code Editor themes (e.g. prism-okaidia) hardcode a font-family declaration directly on .prism-code-editor. Because bslib's code-editor.css only set font-family on bslib-code-editor .code-editor and its textarea/pre/code children — not on .prism-code-editor itself — the theme's font won the cascade and caused the caret to visually drift to the right of the actual text insertion point.

The fix adds .prism-code-editor to the existing font selector in inst/components/dist/code-editor.css, ensuring bslib's monospace font overrides whatever the theme injects.

Verification

Open input_code_editor() with any non-default theme and type a long line of code. The caret should stay visually aligned with the insertion point throughout.

library(shiny)
library(bslib)

ui <- page_fillable(
  input_code_editor("code", value = "f <- function(x) x + 1\nf(42)", language = "r", theme = "prism-okaidia")
)

shinyApp(ui, \(i, o, s) {})

[cpsievert]

Looks like 7c3b7d3 undid the intended fix, maybe since it's a "distributed file"?