Move the en-GB language plugin into the core

We need to have the default language baked into the core so that we
don’t have problems if no language plugins are available.

core/language/en-GB.tid

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+title: $:/languages/en-GB
+name: en-GB
+description: English (British)
+author: JeremyRuston
+core-version: >=5.0.0"
+
+Stub pseudo-plugin for the default language

core/language/en-GB/ControlPanel.tids

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+title: $:/lingo/ControlPanel/
+
+Basics/Caption: Basics
+Basics/Version: ~TiddlyWiki version
+Basics/Title: Title of this ~TiddlyWiki
+Basics/Subtitle: Subtitle
+Basics/Username: Username for signing edits
+Basics/AnimDuration: Animation duration
+Basics/DefaultTiddlers: Default tiddlers
+Basics/DefaultTiddlers/Prompt: Choose which tiddlers are displayed at startup:
+Basics/DefaultTiddlers/Hint: Use &#91;&#91;double square brackets&#93;&#93; for titles with spaces. Or you can choose to <$button set="$:/DefaultTiddlers" setTo="[list[$:/StoryList]]">retain story ordering</$button>
+Basics/Tiddlers: Number of tiddlers
+Basics/Tags: Number of tags
+Basics/SystemTiddlers: Number of system tiddlers
+Basics/ShadowTiddlers: Number of shadow tiddlers
+Basics/OverriddenShadowTiddlers: Number of overridden shadow tiddlers
+Appearance/Caption: Appearance

core/language/en-GB/languageprompt.tid

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+title: $:/lingo/ControlPanel/Language/prompt
+
+Hello! Current language: