refactor: invert some conditionals for better readability

Summary

Small refactor to invert some conditionals/if statements for better readability

handful of refactors I'm making as I'm now very familiar with the codebase to make it easier to work with and more approachable to new contributors as well

a few if (cond) { big block } return could be inverted to if (!cond) return then the block un-indented instead
- generally speaking, this makes it a lot more readable (less indentation, etc) and follows the existing code style in much of the codebase, where it's a few quick one-line ifs and then just the rest of the code
shorten the resolvedFileName conditional by using optional chaining
- prefer the simpler x?.y over the older x && x.y syntax
add a resolved variable for less repetition of the whole statement
add a comment to the pathNormalize line about why it's used in that one place and link to the longer description in the PR as well
shorten comment about useTsconfigDeclarationDir so it doesn't take up so much space or look so important as a result
- it's easier to read this way and I made the explanation less verbose and quicker to read too
- already have comments and lines in the codebase longer than 80 chars, so that's not a guideline that's followed anywhere else
remove the else there and just add an early return instead, similar to the inverted conditionals above
- similarly makes it less unindented, more readable etc

This is unfortunately going to merge conflict with #334 as one of the conditionals is changed there as well, so I'll need to rebase this once that's fixed
Ignoring whitespace changes makes this PR easier to read