Function superlative

This function returns the superlative form of an adjective.

Uses irregular forms for common words (good -> best) and heuristic rules for regular adjectives:

• 1 syllable: add -est (fast -> fastest)
• 1 syllable ending in CVC: double consonant (big -> biggest)
• 2 syllables ending in -y: change y to -iest (happy -> happiest)
• 2 syllables ending in -e: add -st (large -> largest)
• 2 syllables ending in -le or -ow: add -est (simple -> simplest, narrow -> narrowest)
• 3+ syllables: use "most" (beautiful -> most beautiful)