unicode: add CategoryAliases, Cn, LC

CategoryAliases is for regexp to use, for things like \p{Letter} as an alias for \p{L}.
Cn and LC are special-case categories that were never implemented
but should have been.

These changes were generated by the updated generator in CL 641395.

Fixes #70780.

Change-Id: Ibba20ff76191c8ae9631ac5ba19965790fe0cc81
Reviewed-on: https://go-review.googlesource.com/c/go/+/641376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

api/next/70780.txt

@@ -0,0 +1,3 @@
+pkg unicode, var CategoryAliases map[string]string #70780
+pkg unicode, var Cn *RangeTable #70780
+pkg unicode, var LC *RangeTable #70780

doc/next/6-stdlib/99-minor/unicode/70780.md

@@ -0,0 +1,4 @@
+The new [CategoryAliases] map provides access to category alias names, such as “Letter” for “L”.
+The new categories [Cn] and [LC] define unassigned codepoints and cased letters, respectively.
+These have always been defined by Unicode but were inadvertently omitted in earlier versions of Go.
+The [C] category now includes [Cn], meaning it has added all unassigned code points.

src/unicode/script_test.go

@@ -52,6 +52,10 @@ var inCategoryTest = []T{
 	{0x00bb, "P"},
 	{0x00a2, "S"},
 	{0x00a0, "Z"},
+	{0x0065, "LC"},
+	// Unassigned
+	{0x0378, "Cn"},
+	{0x0378, "C"},
 }
 
 var inPropTest = []T{