Support custom `language` editor support / syntax highlighting

[nikku]

Describe the Problem

When users define templates they may define fields representing custom language data (#858).

Describe the solution you want

Be able to plug-in custom editing support for Text#language attached

• Syntax highlighting
• Line indention
• Editor completion

Alternatives

Keep a stock editing experience (monospace, auto-resizable textarea). We'll miss out on important language support in that case.

Additional Context

Brought up in the context of camunda/connectors#202.

[nikku]

Some ideas sketched how we can use the language field to plug-in other editors.

[nikku]

Moving this to ready to discuss with the team. Having this extension point would unblock others; they could contribute custom editors (i.e. GraphQL, ...).