📤 Refactor exports for more flexibility

[fwkoch]

Previously, when defining exports, you had to specify format:

---
exports:
  - format: pdf
---

This is redundant with extensions...

---
exports:
  - format: pdf
    output: index.pdf
---

...and templates...

---
exports:
  - format: typst
    output: lapreprint-typst
---

There is also ambiguity around what format: pdf means, e.g. if I want a pdf built with typst, this failed since pdf was synonymous with tex build:

---
exports:
  - format: pdf
    output: lapreprint-typst
---

This PR refactors the export collection logic to be more human-friendly.

• Format can now be determined from format, output, or template, redundancy is not required
• format: pdf is more flexible and can result in a typst build if typst template or file extension are used
• Export may now specify zip: true - previously the only way to zip outputs was provide an output with .zip extension.

It also centralizes the logic into format-agnostic functions, so the collectTexExportOptions, collectDocxExportOptions, etc functions are deprecated. Also previously there was some conflation between CLI-provided export options and file-provided export options... this has been separated a bit more cleanly, so all the CLI-provided options are handled in build, not in collectExportOptions...

👀 ALSO 👀 (Definitely not too much here. Definitely not.) This addresses #1021 by throwing an error when typst executable is not available, rather than silently carrying on and encountering other inexplicable errors...

[fwkoch]

I need to add some unit tests at least on these resolve* functions before merging this...