Implement optional patterns

[kyouko-taiga]

Now we can write the following:

public fun main() {
  let x: Optional<Int> = 42 as _
  if let y? = x { print(y) }
}

[codecov]

Codecov Report

Attention: Patch coverage is 98.27586% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.88%. Comparing base (dff9eca) to head (a962208).
Report is 5 commits behind head on main.

Files	Patch %	Lines
Sources/FrontEnd/TypeChecking/TypeChecker.swift	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1569      +/-   ##
==========================================
+ Coverage   87.83%   87.88%   +0.04%     
==========================================
  Files         373      374       +1     
  Lines       22627    22665      +38     
==========================================
+ Hits        19875    19919      +44     
+ Misses       2752     2746       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.