Make weight conversion matcher more robust

[hargup]

Summary of changes:

• Remove the requirement of "convert" in the query
• Match, lb and kg
• Refactor code for easier matching of synonyms.

[hargup]

@N-Shar-ma

[hargup]

@sahil-shubham addressed all the comments, also changed to replaceSynonyms to match full words instead of substring. For example, the earlier implementation would have matched gram in anagram, but not in present implementation.