Previously, there was a single parsing rule for "bare words" that
applied to both internal and external commands.

This meant that, because `cargo +nightly` needed to work, we needed to
add `+` as a valid character in bare words.

The number of characters continued to grow, and the situation was
becoming untenable. The current strategy would eventually eat up all
syntax and make it impossible to add syntax like `@foo` to internal
commands.

This patch significantly restricts bare words and introduces a new token
type (`ExternalWord`). An `ExternalWord` expands to an error in the
internal syntax, but expands to a bare word in the external syntax.

`ExternalWords` are highlighted in grey in the shell.

Bare words now represent literal file names, and globs are a different
syntax shape called "Pattern". This allows commands like `cp` to ask for
a pattern as a source and a literal file as a target.

This also means that attempting to pass a glob to a command that expects
a literal path will produce an error.