Add ConvertTo/FromBase64 property functions

This is a follow up after dotnet/msbuild#7554

Author: stan-sz

docs/msbuild/property-functions.md

@@ -174,7 +174,9 @@ Here is a list of MSBuild property functions:
 |string GetPathOfFileAbove(string file, string startingDirectory)|Searches for and returns the full path to a file in the directory structure above the current build file's location, or based on `startingDirectory`, if specified.|
 |GetDirectoryNameOfFileAbove(string startingDirectory, string fileName)|Locate and return the directory of a file in either the directory specified or a location in the directory structure above that directory.|
 |string MakeRelative(string basePath, string path)|Makes `path` relative to `basePath`. `basePath` must be an absolute directory. If `path` cannot be made relative, it is returned verbatim. Similar to `Uri.MakeRelativeUri`.|
-|string ValueOrDefault(string conditionValue, string defaultValue)|Return the string in parameter 'defaultValue' only if parameter 'conditionValue' is empty, else, return the value conditionValue.|
+|string ValueOrDefault(string conditionValue, string defaultValue)|Returns the string in parameter 'defaultValue' only if parameter 'conditionValue' is empty, else, return the value conditionValue.|
+|string ConvertToBase64(string toEncode)|Returns the string after converting all bytes to base 64 (alphanumeric characters plus '+' and '/'), ending in one or two '='.|
+|string ConvertFromBase64(string toDecode)|Returns the string after converting from base 64 (alphanumeric characters plus '+' and '/'), ending in one or two '='.|
 
 ## Nested property functions