[dcl.init] Explicitly define "initializer expression"

In the current wording, [dcl.init] often refers to an "initializer expression" without clearly defining what that would be. In most initialization contexts, an initialization expression does not exist, and only exists when using the parenthized expression-list form of an initializer with a single expression, or the = form of a brace-or-equals-initializer. While with much effort it can be deduced from the wording (with knowledge of expected behavior), it should be explicitly defined to increase readability.

Start by reading the [dcl.init] wording and the uses of "initializer expression" described in the issue. Define the term explicitly in that section while preserving the stated initialization cases, then verify that each existing use is covered and the wording is clearer.