Require word boundary in alias reminder prefix match

[shinenelson]

The alias reminder compared the typed command against an alias's
expansion using a plain string prefix check, so any typed command
that happened to start with the expansion text matched — even when
the remaining characters weren't a separate argument. An alias like
...='cd ../../' then falsely fired for cd ../../another/path, a path
the alias can't reach, since using it would pass "../../" and
"another/path" as two separate arguments to cd instead of one
concatenated path. Require the match to land on a word boundary
(end of string or a following space) in bash, zsh, fish, and
nushell.