Inner command words highlighting disregards allowed types

[danielshahaf]

In the commands builtin foo, command foo, and sudo foo, the inner command word foo should be looked up only as a builtin or external command, respectively.

After builtin aliases, function names, hashed command names, and absolute paths should be red even if they're otherwise valid; similar restrictions apply after command and external commands that are precommands (e.g., sudo/doas, env, ssh, xargs, chroot/jexec, strace, setsid, stdbuf, etc). We needn't enumerate them; any known precommand [see $precommand_options] that's itself an external command — that is, isn't shadowed by a function or alias — can only be followed by external commands.

This should be fairly easy to fix by making the setting and testing of the :start: state more fine-grained.

• Add test cases of the form builtin foo.
• Add test cases of the (somewhat misleading) form builtin command foo.
• Add test cases with precommands that are external commands.

[danielshahaf]

(GitHub's UI doesn't show that I pushed the tests to this repository as well: bc3f77f)

[danielshahaf]

Cross-referencing #249 #541 #545, which are all variants of this.

[danielshahaf]

Also #461.