feat: enhance is and as keywords

[Chronostasys]

close #436

• enhance is, as
• add if let
• add impl

[codecov]

Codecov Report

Attention: Patch coverage is 75.54945% with 89 lines in your changes missing coverage. Please review.

Project coverage is 85.05%. Comparing base (7afd45c) to head (72361b2).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ast/ctx/builtins.rs	0.00%	45 Missing ⚠️
src/ast/node/cast.rs	84.42%	19 Missing ⚠️
src/ast/fmt.rs	0.00%	15 Missing ⚠️
src/ast/node/control.rs	93.63%	7 Missing ⚠️
src/ast/node/interface.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
- Coverage   85.19%   85.05%   -0.15%     
==========================================
  Files          99       99              
  Lines       25853    26147     +294     
==========================================
+ Hits        22026    22239     +213     
- Misses       3827     3908      +81     

Files with missing lines	Coverage Δ
src/ast/compiler.rs	73.68% <100.00%> (ø)
src/ast/compiler/progress.rs	100.00% <100.00%> (ø)
src/ast/diag.rs	69.02% <ø> (ø)
src/ast/node/mod.rs	83.70% <ø> (ø)
src/nomparser/cast.rs	100.00% <100.00%> (ø)
src/nomparser/control.rs	99.06% <100.00%> (+<0.01%)	⬆️
src/nomparser/expression.rs	99.68% <100.00%> (+<0.01%)	⬆️
src/nomparser/helper.rs	98.17% <100.00%> (ø)
src/nomparser/identifier.rs	97.34% <100.00%> (-0.03%)	⬇️
src/nomparser/types.rs	99.09% <100.00%> (ø)
... and 5 more

... and 2 files with indirect coverage changes

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[Chronostasys]

Docs & test cases might be added later next week.