Make eshell history expansion more like bash (Bug#29821)

Make eshell history expansion more like bash (Bug#29821)

- Prevent expansion of quick substitutions when the initial "^" is not
  at start of line (Bug#29157).
- Allow spaces inside substitutions, so "^foo bar^baz" works.
- Allow trailing characters after substitution, so "^foo^bar^trailing"
  works.
- Throw an error when substitution does not match.

* lisp/eshell/em-hist.el (eshell-expand-history-references): Expand
history substitution before other types of expansions, and expand them
with the whole line.
(eshell-history-substitution): New function to expand only
substitutions, taking in the entire typed line rather than individual
arguments.