Support numeric font-weight, bolder and lighter, and font-size: larger and smaller

Part of #520 (step 24).

Measured on gravitypdf at d660a410, PHP 8.5.4. All of these are ignored:

Rule	Browser	mPDF
`p{font-weight:700}`	bold	normal
`p{font-weight:600}`	bold	normal
`p{font-weight:bolder}`	bold	normal
`div{font-size:20pt} p{font-size:larger}`	24pt	11pt
`div{font-size:20pt} p{font-size:smaller}`	about 16.7pt	11pt

setCSS() knows only bold and normal for font-weight, and only absolute sizes, keywords such as small, and lengths for font-size.

Fix.

font-weight 100–900: 600 and above are bold, and lower weights normal, while a family has two weights. bolder and lighter are computed from the parent's weight, using the table in CSS Fonts.
font-size: larger and smaller: the parent's size multiplied or divided by 1.2.

Tests. Each value, and each nested two deep.

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