support object index minify

[LeuisKen]

input case:

var arguments=[];(function(){console.log(arguments["1"])})("bar",42);

esbuild output:

var arguments=[];(function(){console.log(arguments["1"])})("bar",42);

terser output:

var arguments=[];!function(){console.log(arguments[1])}("bar",42);

By diff this cases, we could find terser could minify the index, and use ! instead of () to save more bytes. I find the index minify source code is here: https://github.com/terser/terser/blob/master/lib/compress/index.js#L4128

I hope esbuild could support this like terser to make the minification better.

[evanw]

I added minification of integers inside computed property strings. Doing other non-integer numbers seems risky because esbuild is implemented in Go and Go's number-to-string algorithm could hypothetically be slightly different than JavaScript's in subtle ways, which could potentially cause a miscompilation. I declined to do the () to ! transformation because it's typically a very, very small win and doing it correctly adds code complexity.