Boost search results where search string is at the beginning.

[romainruaud]

Is your feature request related to a problem? Please describe.
Main use case is the following :

• I search for "orange" because I want to see the fruit.
• but I mainly see "orange juice" or "gingerbread with orange and pumpkin"

Describe the solution you'd like
I'd like to be able to tell the engine that :

• if the search string is found at the beginning, it must have a better score than when it's fetch inside the string.
• if the search string is "the only word searched", Eg the product name is "orange" and I searched for "orange", it must have an higher score.

Describe alternatives you've considered
Main alternative is to create a boost like :

• search query is "orange".
• Boost products belonging to "fruits" category.

But it's quite a pain to do with a large catalog with many categories.

[romainruaud]

Fixed by #2886