Add custom completions to CodeMirror

[Jepson2k]

Motivation

Embedding ui.codemirror as an in-app code editor often needs to surface a list of host-application-specific identifiers (functions, variables, keywords) in the autocomplete dropdown — e.g. an in-app scripting environment that wants to expose its own API to the user. CodeMirror 6's @codemirror/autocomplete package supports this via autocompletion({override: ...}), but it isn't bundled in the current dist and there's no Python API for it. This PR adds both.

This is one of several small slices being carved out of #5965 per the splitting discussion on that thread.

Implementation

Python (nicegui/elements/codemirror/codemirror.py):

• CompletionItem TypedDict with required label and NotRequired optional fields mirroring CM6's Completion surface (camelCase to round-trip verbatim): displayLabel, detail, info, apply (the text actually inserted on accept; defaults to label), type, boost (sort weight, -99..99), commitCharacters, section.
• type is constrained to COMPLETION_ICON_TYPE, a Literal over CM6's 12 built-in icon names (class, constant, enum, function, interface, keyword, method, namespace, property, text, type, variable), matching the existing SUPPORTED_LANGUAGES / SUPPORTED_THEMES precedent in the same file.
• custom_completions: list[CompletionItem] | None = DEFAULT_PROP | None constructor kwarg, custom_completions property + setter, set_custom_completions(...) method. The kwarg uses the DEFAULT_PROP | None pattern so per-instance defaults compose with the rest of the constructor.

JavaScript (nicegui/elements/codemirror/codemirror.js):

• New customCompletions: Array prop with a Vue watcher that delegates to setCustomCompletions(...) so changes from the Python side propagate without mounting a new editor.
• setCustomCompletions(...) builds an autocompletion({override: [source]}) extension that filters entries by label-prefix against the word before the cursor, then dispatches it through a CM6 Compartment so the source can be replaced after mount. Only fields the user supplied are forwarded to CM6 — no forced type: 'function' default and no padded empty detail/info strings.
• New completionsConfig Compartment slot in the editor's extension list. mounted() initializes the Compartment and applies any initial customCompletions once the editor exists.

Bundle (nicegui/elements/codemirror/src/index.mjs, nicegui/elements/codemirror/dist/):

• Re-export @codemirror/autocomplete from src/index.mjs so autocompletion ends up in the dist bundle. @codemirror/autocomplete was already a transitive dep of the codemirror meta-package, so package.json doesn't change.
• Bundle rebuilt via npm run clean && npm run build.

Tests (tests/test_codemirror.py):

• test_custom_completions types foo into an editor whose initial custom_completions includes two foo_* entries (with different boost values, one with displayLabel) plus an unrelated qux, then asserts: (a) only the two matching entries render, (b) the higher-boost entry sorts first, (c) displayLabel overrides the visible label while label is still used for matching.
• test_set_custom_completions_replaces_initial verifies that calling set_custom_completions after mount replaces the initial source (typing matches the new entries, not the original).

Documentation (website/documentation/content/codemirror_documentation.py):

• A numpy-flavored demo whose entries exercise apply, detail, type, boost, displayLabel, section, and commitCharacters, plus a {'label': 'TODO'} entry to show the no-icon rendering.

Progress

• The PR title is a short phrase starting with a verb like "Add ...", "Fix ...", "Update ...", "Remove ...", etc.
• The implementation is complete. (Otherwise, open a draft PR.)
• This PR does not address a security issue. (Security fixes must be coordinated via the security advisory process before opening a PR.)
• Pytests have been added.
• Documentation has been added.
• No breaking changes to the public API.