Give a more user-friendly diagnostic about the following:

* Trailing tokens within braces, e.g. `${foo() extra}`
* Missing parentheses, e.g. `${foo}`
* Incorrect number of arguments, with a hint about correct usage.

The current messages have the potential to be more accurate; "expected
identifier or string literal" is printed in a few cases where only an
identifier should be expected, and it suggests removing string literals
when that might not solve the problem. Add a new diagnostic for these
kind of errors that gives more context and a better description.

For `count` and `ignore` this should likely be combined with the
diagnositcs for `eat_dollar` to produce a helpful error if they get
anything other than a metavariable first argument. I am planning to do
this in a followup.

wip