feat(prefs): add word characters preference

[royy92]

What

Add a new Preferences field for configuring the word-chars setting.

Why

Issue #2304 points out that users currently have no way to configure the word-chars value from the Preferences window.

This also makes the setting easier to manage when using --save-preferences and --load-preferences.

How

• Add a new word_chars_entry field to guake/data/prefs.glade.
• Load, update, and reset the word-chars setting in guake/prefs.py.
• Add a release note in releasenotes/notes/word-chars-preference-9fc892802ca01441.yaml.

Testing

• Ran make style.
• Ran make check.
• Ran make test (30 tests passed).
• Verified that changing the field updates the word-chars setting.
• Verified that resetting Compatibility preferences restores the default word-chars value.

Screenshot

The new Word characters preference available in the Compatibility section.

Closes #2304