Use a default template instead of separate code path in Help::write_help

[CreepySkeleton]

As @kitlith noticed in #1986 (comment),:

Using a template vs not using a template go through two somewhat different code-paths, right? You don't internally use a template unless the user provides one.

In other words, instead of using some sort of hardcoded template, we use a separate function that acts like a hardcoded template.

I think that it worth exploring simply using a hardcoded template instead of a separate code path. We may also export this template as pub static: &str DEFAULT_TEMPLATE, but I'm opposed to considering precise contents of the default template as an API contract as we may (and probably will) tweak it here and there.

Related code:

clap/src/output/help.rs

Lines 110 to 115 in 9fc8ec9

	if let Some(h) = self.parser.app.help_str {
	self.none(h).map_err(Error::from)?;
	} else if let Some(tmpl) = self.parser.app.template {
	self.write_templated_help(tmpl)?;
	} else {
	self.write_default_help()?;

The function in question:

clap/src/output/help.rs

Line 821 in 9fc8ec9

pub(crate) fn write_default_help(&mut self) -> ClapResult<()> {

[kitlith]

I agree. The precise contents of that template should be able to change. The guarantee I was going for is not "this is the template that will always produce the default output" but "this is the template that produces the default output for this version".