WebGPU: improve code style in library_webgpu.js

[kainino0x]

Tracking issue.

• Avoid string property access in library_webgpu.js. NFC #21454
• Use let/const instead of var? Need to write a code size test first to find out whether Closure Compiler handles these well. See Replace var with const/let in library_(html5_)webgpu.js #21488
• Change dictionary literals to either use single quotes (more consistent style) or no quotes. Need to check (a) whether Closure can understand these and avoid minifying them and (b) whether removing the quotes would improve Closure output code size. See WebGPU: support Premultiplied canvas output #22494 (comment)
• Do something to reduce maintenance overhead of the various string<->int tables. (Dawn's fork does something about this, but it currently relies on Dawn's code generator.)
• ?

[kainino0x]

Assigning to myself for tracking but I am not working on it right now.