Update escape_latex() for better rendering

[rich-iannone]

Certain replacements for LaTeX escapes that supply a command will render better (i.e., not include unwanted space characters) if they use {} (e.g., \textasciitilde{} instead of \textasciitilde ). To that end, this PR enhances the escape_latex() function to make the replacements render LaTeX text more accurately.

[jcheng5]

latex_special_chars <- c(
  "\\" = "\\textbackslash{}",
  "&" = "\\&",
  "~" = "\\textasciitilde{}",
  "^" = "\\textasciicircum{}"
)

escape_latex <- function(text) {
  m <- gregexec("[\\\\&%$#_{}~^]", text)
  special_chars <- regmatches(text, m)
  escaped_chars <- lapply(special_chars, function(x) {
    latex_special_chars[x]
  })
  regmatches(text, m) <- escaped_chars
  text
}