[Expression] Add merge prebuilt function for merging JSON object

[cosmicshuai]

closes: #2468

Parity of botbuilder-dotnet PR:
microsoft/botbuilder-dotnet#4075

merge function can take JSON object as input.
merge(jobject1, jobject2, ...)

for example:

json1 = @"{
'FirstName': 'John',
'LastName': 'Smith',
'Enabled': false,
'Roles': [ 'User' ]
}"
json2 =@"{
'Enabled': true,
'Roles': [ 'User', 'Admin' ]
}"
string(merge(json(json1), json(json2))) will return

{"FirstName":"John","LastName":"Smith","Enabled":true,"Roles":["User","Admin"]}

[coveralls]

Pull Request Test Coverage Report for Build 140201

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.07%) to 80.888%

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Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%

Totals
Change from base Build 139946:	-0.07%
Covered Lines:	12906
Relevant Lines:	15236

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💛 - Coveralls

[chrimc62]

To many extra lines and remove tODO

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Refers to: libraries/adaptive-expressions/src/expressionType.ts:161 in 4a11550. [](commit_id = 4a11550, deletion_comment = False)

[chrimc62]

🕐

[chrimc62]