feat: drop unused local variables

[sapphi-red]

This PR partially implements dropping unused local variables.
Multi-level dropping (for example: a is unused and b depends on a) is not implemented.

This is useful for some cases like minifying IIFE wrapped output.

Example

Input

(function () {
  'use strict';

  const const1 = 0;
  let let1 = 0;
  var var1 = 0;

  const const2 = 0;
  let let2 = 0;
  var var2 = 0;
  console.log(const2, let2, var2);

})();

Current Output

(function() {
  "use strict";
  let let1 = 0;
  var var1 = 0;
  const const2 = 0;
  let let2 = 0;
  var var2 = 0;
  console.log(const2, let2, var2);
})();

New Output

(function() {
  "use strict";
  const const2 = 0;
  let let2 = 0;
  var var2 = 0;
  console.log(const2, let2, var2);
})();

[sapphi-red]

This test is related to TDZ.
https://github.com/mishoo/UglifyJS/blob/4c227cc6bd795278c085a6b6dfd40a960ec35698/test/compress/let.js#L1486-L1492

[sapphi-red]

If this costs too much, I think this condition could be p.currentScope.Parent == p.moduleScope && !p.currentScope.ContainsDirectEval instead.

[sapphi-red]

This test fails without this PR.
const v = 0; is dropped at here.

esbuild/internal/js_parser/js_parser.go

Lines 9312 to 9317 in 698898f

	// Only keep this declaration if it's top-level or exported (which
	// could be in a nested TypeScript namespace), otherwise erase it
	if p.currentScope.Parent == nil || s.IsExport {
	s.Decls[end] = d
	end++
	}

Created an issue: #2361

[unbyte]

This PR breaks var hoist.

before

function test(v) {
  var v = 3
  return arguments[0]
}

test(1)
// output: 3

after

function test(v) {
  return arguments[0]
}

test(1)
// output: 1

[sapphi-red]

@unbyte That's a good catch.
But I will not campion this PR. This is because this PR is no longer essential for me.
Feel free to create a PR that supersedes this one. 😃