rebrand $/.before as .prematch and .after as .postmatch

I have been told on #perl6 that the old names are unclear, this seems to be
the new consensus

Author: moritz

S05-regex.pod

@@ -2837,16 +2837,16 @@ the match. For example:
 
 The currently defined methods are
 
-    $/.from     # the initial match position
-    $/.to       # the final match position
-    $/.chars    # $/.to - $/.from
-    $/.orig     # the original match string
-    $/.Str      # substr($/.orig, $/.from, $/.chars)
-    $/.ast      # the abstract result associated with this node
-    $/.caps     # sequential captures
-    $/.chunks   # sequential tokenization
-    $/.before   # text before the match
-    $/.after    # text after the match
+    $/.from      # the initial match position
+    $/.to        # the final match position
+    $/.chars     # $/.to - $/.from
+    $/.orig      # the original match string
+    $/.Str       # substr($/.orig, $/.from, $/.chars)
+    $/.ast       # the abstract result associated with this node
+    $/.caps      # sequential captures
+    $/.chunks    # sequential tokenization
+    $/.prematch  # $/.orig.substr(0, $/.from)
+    $/.postmatch # $/.orig.substr($/.to)
 
 Within the regex the current match state C<$¢> also provides