Add CSS function completion provider

[vanillajonathan]

PR Checklist

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• I have read, and I am following the Contributor guide
• I have read, and I understand the Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

I accidentally messed up #1951 when I tried to rebase it around the changes in the main branch.

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.32%.

Quality metrics	Before	After	Change
Complexity	2.46 ⭐	2.29 ⭐	-0.17 👍
Method Length	50.17 ⭐	48.55 ⭐	-1.62 👍
Working memory	4.61 ⭐	4.57 ⭐	-0.04 👍
Quality	80.64% ⭐	80.96% ⭐	0.32% 👍

Other metrics	Before	After	Change
Lines	366	431	65

Changed files	Quality Before	Quality After	Quality Change
gaphor/ui/csscompletion.py	82.73% ⭐	82.37% ⭐	-0.36% 👎
gaphor/ui/elementeditor.py	80.07% ⭐	80.21% ⭐	0.14% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
gaphor/ui/elementeditor.py	EditorStack.show_no_item_selected	4 ⭐	131 😞		67.00% 🙂	Try splitting into smaller methods
gaphor/ui/csscompletion.py	CssFunctionCompletionProvider.do_refilter	4 ⭐	74 🙂	10 😞	68.77% 🙂	Extract out complex expressions
gaphor/ui/csscompletion.py	CssPropertyCompletionProvider.do_refilter	4 ⭐	74 🙂	10 😞	68.77% 🙂	Extract out complex expressions
gaphor/ui/elementeditor.py	PreferencesStack.open	2 ⭐	124 😞	6 ⭐	72.97% 🙂	Try splitting into smaller methods

Legend and Explanation

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• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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[amolenaar]

Nice! Thanks @vanillajonathan!