Consistently use "dynamic type name" and "expression" instead of "variable value" in TypedConfigManager's terminology

Problem/Motivation

In a number of configuration system/configuration schema issue now, the confusing nature of the term "variable value" has come up.

Quoting \Drupal\Core\Config\TypedConfigManager::replaceVariable():

  /**
   * Replaces variable values in included names with configuration data.
   *
   * Variable values are nested configuration keys that will be replaced by
   * their value or some of these special strings:
   * - '%key', will be replaced by the element's key.
   * - '%parent', to reference the parent element.
   * - '%type', to reference the schema definition type. Can only be used in
   *   combination with %parent.
   *
   * There may be nested configuration keys separated by dots or more complex
   * patterns like '%parent.name' which references the 'name' value of the
   * parent element.
…

Steps to reproduce

N/A

Proposed resolution

Rename "variable value" to "dynamic placeholder".

Rename "variable value" to "dynamic type name", since that terminology was +1'd by configuration system maintainer Alex Pott in #2392057: Config schema fails to expand dynamic top-level types. But there's a distinction between the entire dynamic type name and the parts in there that are dynamic. I propose the word "expression", since it must be evaluated.

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

N/A