Add more specific syntax schemes for AvalonEdit

[thomaslevesque]

Fixes #32 Introduce 3 syntax schemes:

• JSON (was previously using Javascript, which didn't work very well because keys and values had the same color)
• CosmosJS : same as the default Javascript syntax from AvalonEdit, with a few additional keywords (async, await, let)
• CosmosSQL : based on the default TSQL syntax from AvalonEdit, but with CosmosDB specific keywords, and without unsupported keywords