Better abstract string concatenation

[TimothyGu]

Due to the way we construct type conversions, oftentimes we see unidiomatic code like

webidl2js/test/__snapshots__/test.js.snap

Line 167 in 7d45dfc

\\"Failed to execute 'bs' on 'BufferSourceTypes': parameter 1\\" + \\" is not of any supported type.\\"

Ideally we want to merge string literals together and even use template literals.

Related to this is #204, but this seems mostly orthogonal and more tractable.

[domenic]

I wonder if this could be done as a post-processing step using eslint --fix or some more aggressive prettier configuration.

[TimothyGu]

Prettier has a stance of not changing ASTs, so I guess that'd be off. ESLint has no-useless-concat, but sadly no fixer for it. This is easy to do even without a postprocessor though, so maybe we should just write the code.

[ExE-Boss]

I already have code for this in https://github.com/ExE-Boss/webidl2js/tree/refactor/precompute-concatenated-strings from a month ago, but I wasn’t sure whether it was something that was needed, and now it needs rebasing.