Add new eslint rules (no-unnecessary-condition, no-unused-expressions, etc) and tsconfig noUncheckedIndexedAccess

[cmdcolin]

This PR adds some upgrades to linting

specifically

• adds eslint 9, with flat config
• new lint rule @typescript-eslint/no-unnecessary-condition. Adding this lint rule surfaces many "superstitious" instances of if-statements/ternarys/etc. which are just always true or always false, detectable via types
• new lint rule @typescript-eslint/no-confusing-void-expression, avoids returning values from functions where return type void is expected (so many bracketless arrow functions are converted to bracketed ones)
• tsconfig.json rule noUncheckedIndexedAccess:true, which makes accessing objects and arrays e.g. obj[key] assume to T|undefined which means you have to override the setting to produce the desired result. this results in more common usage of non-null assertion in some cases, but generally improves typing in others

[cmdcolin]

also adds eslint rule @typescript-eslint/no-unused-expressions which caught an error in coverage calculations in the methylation mode

diff here shows that there was a "unused expression" where a mutation was intended