Standardize usage of compound words, and document standards in docs style guide

**Your Godot version:**
4.4
**Issue description:**
There are some compound words, like "antialiasing", that appear in the docs in both hyphenated form "anti-aliasing" and in single-word form "antialiasing". For consistent style, we should choose one form and standardize.

Some examples:
- antialiasing / anti-aliasing
- runtime / run-time
- screen space / screen-space
- builtin / built-in / built in (this one depends on context)
- subsurface / sub-surface
- subemitter / sub-emitter
- standalone / stand-alone
- low-level / low level
- high-level / high level
- low-end / low end
- addon / add-on (this one shows up in editor UI and in the `res://addons/` folder, too!)

Resolved and standardized (within the manual):
- Use "antialiasing", not "anti-aliasing". Added to codespell dictionary.
- Use "runtime", not "run-time". In specific cases, "run time" is okay.
- Use "built-in" as an adjective or noun, "built into" as a phrase. Avoid "built in" or "builtin", and avoid "built-in to".
- Use "low-level", "low-end", "high-level", "high-end" as adjectives.

This is a fairly low priority issue. We should not break translations just to change this. However, we should choose which form to use for each compound word, document that choice in the style guide, and change existing usages whenever other changes are being made.

There's also precedent in https://github.com/godotengine/godot-docs/pull/3502, https://github.com/godotengine/godot-docs/pull/6978 of applying changes like this all at once.

**URL to the documentation page (if already existing):**
The whole manual and class reference.