Turn 1-line prose algorithms into step-based algorithms for readability.

This is especially an issue with the IDL to ES conversion algorithms.

For example, turn:

When the \[[DefineOwnProperty]] internal method of a [=named properties object=] is called,
it must return <emu-val>false</emu-val>.

into:

<div algorithm="to invoke the internal [[DefineOwnProperty]] method of named properties object">

    When the \[[DefineOwnProperty]] internal method of a [=named properties object=] is called,
    the following steps are taken:

    1.  Return <emu-val>false</emu-val>.
</div>

For context, see discussion in https://github.com/heycam/webidl/pull/198#issuecomment-255540339 and following comments.