feat(agent): add experimental option: strip auto-closing chars in prompt suffix.

[icycodes]

Complete TAB-268

Add this config to enable experimental feature:

[completion.prompt]
stripAutoClosingCharacters = true

Example:
For this case:

function fib(n) {║}

off(default):

// prompt
{
  prefix: "function fib(n) {",
  suffix: "}\n",
}

on:

// prompt
{
  prefix: "function fib(n) {",
  suffix: "\n",
}

[wsxiaoys]

change naming to experimentalStripAutoClosingCharacters