Better tag errors

[wch]

Prior this change, the errors from tag functions when there are extra commas are pretty useless:

> div("a", "b", )
Error in tag("div", list(...)) : argument is missing, with no default

> tags$div("a", "b", )
Error in tag("div", list(...)) : argument is missing, with no default

After this change:

> div("a", "b",)
Error in div("a", "b",  ) : argument is missing, with no default

> tags$div("a", "b",)
Error in tags$div("a", "b",  ) : argument is missing, with no default

This PR also:

• Programmatically generates the tag functions instead of embedding a literal string in each one.
• Simplifies the directly-exported tag functions (like div) by removing the wrapper function.

[wch]

The change that improves the error message is just that list(...) is evaluated before being passed to the tag() function. To illustrate:

# Old version
f <- function(...) {
  tag("div", list(...))
}

f(1, 2, )
# Error in tag("div", list(...)) : argument is missing, with no default


# New version
g <- function(...) {
  contents <- list(...)
  tag("div", contents)
}

g(1, 2, )
# Error in g(1, 2, ) : argument is missing, with no default