Add macro to inline SVG files

[wout]

Purpose

Adds a macro to inline SVG files at compile time.

Description

SVG logos and icons can be inlined with the inline_svg macro in pages and components:

button
  inline_svg("buttons/round.svg")
end

This will:

• inline the SVG file from src/svgs at compile-time (path configurable with an annotation)
• strip the XML declaration, comments newlines and superfluous whitespace
• optionally (and by default) strip styling attributes so the icons can be styled with CSS (also configurable with an annotation)
• raise a compile-time exception when an SVG file is missing
• add a data-inline-svg attribute with the path of the icon (e.g. <svg data-inline-svg="buttons/round.svg">)

Icons can then be styled with:

[data-inline-svg] {
  fill: none;
  stroke: #222;
  stroke-width: 2px;
}

Or:

[data-inline-svg="buttons/round.svg"] {
  fill: #f03;
}

I realise Lucky does not use annotations a lot, but I think they are helpful in this case. The settings defined by the annotations are reasonable defaults, I think, but if necessary, users can jump in and override the defaults.

This PR implements #1760.

Checklist

• [X] - An issue already exists detailing the issue/or feature request that this PR fixes
• [X] - All specs are formatted with crystal tool format spec src
• [X] - Inline documentation has been added and/or updated
• [X] - Lucky builds on docker with ./script/setup
• [X] - All builds and specs pass on docker with ./script/test

[wout]

Forgot to mention I'll add the necessary documentation on the website as well when this is approved.