add defs and method to create linear gradients

[TengilLejonhjarta]

Something I tend to use a lot with SVG is defs for filters and gradients, would something like this be interesting to add to svg5?

Usage something like this

createSVG(500, 500)
let g1 = createLinearGradient(0, 0, 1, 1, [{offset: 0, color: "red"}, {offset: 100, color: "orange"}])
background(`url(#${g1})`)
render()

[MAKIO135]

Nice addition. Could you do this PR with the dev branch instead of the master? I'll handle it from there

[TengilLejonhjarta]

There we go @MAKIO135! Had some issues with the rebase but a squash and merge should clean that up. Only my additions now, pointing to your dev branch 👍