Implement nested <tspan> support

[Mrfence97]

Addresses issue #123.

Implementation is how you'd expect: using a recursive function to process each <tspan> in turn, the base case being when a child is simply a plain #text element. The context of the parent element is passed down to each child.

Note browsers seem to treat text chunks in a linear manner. That is to say that setting the x/y attribute of a nested <tspan> will impact any subsequent <tspan>s, regardless of whether they're a child, sibling or at a higher level. This effectively means that the collection of TextChunks can be stored as a 1D array, rather than a tree. See the third and fourth tests in the new spec SVG.

Things are complicated by the correct trimming of text when xml:space = "default". In particular, spaces need to be consolidated across elements in each TextChunk and between TextChunks. Browsers also treat xml:space differently:

• Chrome: xml:space can be freely changed and all child elements inherit
• Firefox: xml:space once set to preserve cannot be set back to default in any child element

This PR uses the Chrome convention.

[Mrfence97]

Tests were expected to fail, I haven't pushed updated reference PDFs in case you wanted to review yourself. All are due to a better text trimming/space consolidation implementation:

• display-none-and-visibility-inheritance: space character after the 'visible' is no longer rendered
• title-element: spaces by themselves and spaces around the 'Render Me!` text are no longer rendered
• text-placement: The original reference PDF doesn't match browser renderings of the SVG as some leading/trailing spaces were included. Easiest to see looking at how the first d in 'middle' in text-anchor="middle", two tspans is perfectly aligned with the blue line, but in browser renderings (and svg2pdf after this PR), the d is offset: