fix: Potential memory leak issues in node

[tyutjohn]

Adding this code is very effective in node. maybe fix #216
Before:

After:

When running for a long time, it will have a small increase and then become stable. This may depend on the size of the default value of the resvg construction when it is released. I have not found a way to completely release the memory held by rust, but for most scenarios, this increase should be negligible. At present, this code seems to work well.

[tyutjohn]

I really need it, can anyone take a look at it? Thanks!

[tyutjohn]

I modified my test script because I got from @napi-rs/napi-rs#613 that GC might not be triggered correctly. After a long period of running, the memory has stabilized after a small increase.
That is my test code

/* eslint-disable no-console */
import { readFileSync, writeFileSync } from 'fs'
import { join } from 'path'

import { Resvg } from './index.js'

// Helper function to read an SVG file
function readSvgFile(filePath) {
  return readFileSync(filePath, 'utf8')
}

// Read an SVG file
const svgContent = readSvgFile(join('./example/bbox.svg'))

function logMemoryUsage(i, rss, heapTotal, heapUsed, external) {
  console.log(
    `${i.padStart(8, ' ')},` +
      `${rss.padStart(12, ' ')},` +
      `${heapTotal.padStart(12, ' ')},` +
      `${heapUsed.padStart(12, ' ')},` +
      `${external.padStart(12, ' ')},`,
  )
}

function sleep() {
  return new Promise((resolve) => setTimeout(resolve, 1000))
}

async function simulateMemoryUsage() {
  logMemoryUsage('i', 'rss', 'heapTotal', 'heapUsed', 'external')

  for (let i = 0; i < 1000000; i++) {
    const opts = {
      background: 'rgba(238, 235, 230, .9)',
      fitTo: {
        mode: 'width',
        value: 1200,
      },
      font: {
        fontFiles: ['./example/SourceHanSerifCN-Light-subset.ttf'],
        loadSystemFonts: false,
      },
    }
    const resvg = new Resvg(svgContent, opts)
    const pngData = resvg.render()
    const pngBuffer = pngData.asPng()
    pngData.free()
    resvg.free()

    if (i % 10 === 0) {
      // NOTE: Do GC before measuring memory usage because the garbage is not measured.
      global.gc()
      await sleep()
      const usage = process.memoryUsage()

      logMemoryUsage(
        i.toString(),
        usage.rss.toString(),
        usage.heapTotal.toString(),
        usage.heapUsed.toString(),
        usage.external.toString(),
      )
    }
  }
}
simulateMemoryUsage()