Add matchedSubstrings to spellCheck interface

[alextaing]

Raiden needs access to matchedSubstrings from the spellCheck part of the Search API response. Since changing correctedQuery from a string to an object would be a breaking change, we added a new property to the spellCheck interface that holds the matchedSubstrings. matchedSubstrings was also added to the Core response when the spellCheck object is created in createSpellCheck.

J=SLAP-2274
TEST=manual

It was verified that when spellCheck was provided in the Core response, that matchedSubstrings was also provided as its own property.

[coveralls]

Pull Request Test Coverage Report for Build 2739777204

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.826%

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Totals
Change from base Build 2691813093:	0.0%
Covered Lines:	210
Relevant Lines:	227

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💛 - Coveralls

[yen-tt]

to confirm, you want to publish a beta version right? This should point to develop instead of master