update `layerRect()` to handle inconsistent `leading` and `spaceBefore`/`spaceAfter` paragraph props

[fartinmartin]

Current layerRect() will calculate lineGap based on sourceText.style.leading and multiply by number of lines:

https://github.com/motiondeveloper/aefunctions/blob/a6a777177fe0e0acb5451a0f0f265fecd41153a1/src/index.ts#L300-L307

But, if a text layer has per-character styles applied (via UI, or via expressions from something like style-parser) we'd need to calculate lineGap based on the styles at each line's starting character index value.

This PR introduces a new option consistentLeading that when set to false will calculate as described. (Open to a better option name/API here, haha!)

Additionally, the spaceBefore and spaceAfter paragraph props can affect the height of a given text layer. This PR adds the appropriate height given said paragraph spacing as well.

Here's an example project to check out: aefunctions-layerRect.zip

Note: In this PR, I no longer use textCount() to determine numLines. I found the regexp in this PR to be more consistent/expected—let me know if it breaks an edge case I haven't considered!