Add pseudo-tokenization of `plaintext` language documents

[zm-cttae]

⚡️️ Feature Request

• [x] I'm using the latest version of vscode-textmate-languageservice available.
• [x] I searched existing issues, open & closed. Yes, my feature request is new.

Is your feature request related to a problem?

• As a: developer tokenising arbitrary documents that aren't registered
• I want to: receive a semi-valid facsimile of plaintext tokenization
• So that: I can universally tokenize and parse all VS Code documents without errors

Describe the solution you'd like

Add semi-valid line tokenization and API results for plaintext documents and language.

Describe alternatives you've considered

Document the error as it is technically valid to vscode-textmate result in core app.

/label ~feature-request

[zm-cttae]

There is an engineering problem here in that the LanguageDefinition.aliases[0] value is localised. Its actually the plainText.alias NLS string, which makes things more challenging.

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As with most extensions in the Marketplace it would be best to just hardcode this to English for now.

[zm-cttae]

Thanks to a Good Samaritan on the extdev Slack, we can automate the i18n of the label:

https://github.com/microsoft/vscode-loc/blob/v1.82.2023090609/i18n/vscode-language-pack-es/translations/main.i18n.json#L804C5-L804C20