Editorial: Add definition for the Pattern String type

Add a simple definition for Pattern Strings and rename some parameters,
so they link to the new definition.

spec/listformat.html

@@ -235,7 +235,7 @@ <h1>Abstract Operations for ListFormat Objects</h1>
     <emu-clause id="sec-deconstructpattern" type="abstract operation">
       <h1>
         DeconstructPattern (
-          _pattern_: a String,
+          _pattern_: a Pattern String,
           _placeables_: a Record,
         ): a List
       </h1>

spec/locales-currencies-tz.html

@@ -2,7 +2,7 @@
   <h1>Identification of Locales, Currencies, Time Zones, Measurement Units, Numbering Systems, Collations, and Calendars</h1>
 
   <p>
-    This clause describes the String values used in this specification to identify locales, currencies, time zones, measurement units, numbering systems, collations, and calendars.
+    This clause describes the String values used in this specification to identify locales, currencies, time zones, measurement units, numbering systems, collations, calendars, and pattern strings.
   </p>
 
   <emu-clause id="sec-case-sensitivity-and-case-mapping">
@@ -414,4 +414,11 @@ <h1>
       </dl>
     </emu-clause>
   </emu-clause>
+
+  <emu-clause id="sec-pattern-string-types">
+    <h1>Pattern String Types</h1>
+    <p>
+      <dfn>Pattern String</dfn> is a String value which contains zero or more substrings of the form *"{name}"*. The syntax of the abstract pattern strings is an implementation detail and is not exposed to users of ECMA-402.
+    </p>
+  </emu-clause>
 </emu-clause>

spec/negotiation.html

@@ -471,14 +471,13 @@ <h1>
     <emu-clause id="sec-partitionpattern" type="abstract operation">
       <h1>
         PartitionPattern (
-          _pattern_: a String,
+          _pattern_: a Pattern String,
         ): a List of Records with fields [[Type]] (a String) and [[Value]] (a String or *undefined*)
       </h1>
       <dl class="header">
         <dt>description</dt>
         <dd>
           The [[Value]] field will be a String value if [[Type]] is *"literal"*, and *undefined* otherwise.
-          The syntax of the abstract pattern strings is an implementation detail and is not exposed to users of ECMA-402.
         </dd>
       </dl>
       <emu-alg>

spec/relativetimeformat.html

@@ -343,7 +343,7 @@ <h1>
     <emu-clause id="sec-makepartslist" type="abstract operation">
       <h1>
         MakePartsList (
-          _pattern_: a pattern String,
+          _pattern_: a Pattern String,
           _unit_: a String,
           _parts_: a List of Records representing a formatted Number,
         ): a List of Records with fields [[Type]] (a String), [[Value]] (a String), and [[Unit]] (a String or ~empty~)