fix(syntaxes): variable token for @let binding

[dannymcgee]

This PR tweaks the TM scopes applied to @let declarations to tokenize the bound identifier as a variable. This brings the grammar into closer alignment with, e.g., TypeScript's TM grammar.

Changes made

• The bound identifier gets its own scope, variable.other.constant.ng.
• meta.definition.variable.ng scope now covers the entire declaration instead of just the identifier and the initializer expression.

Screenshots

Before:

After:

Note

I went with variable.other.constant.ng instead of variable.other.readwrite.ng because it's technically correct (you cannot reassign a @let binding), but it does make it inconsistent with the rest of the grammar, which uses readwrite everywhere. I'd be happy to change it if there's a preference for consistency over correctness. My daily driver theme (and likely most themes on the marketplace) doesn't distinguish between the two, but the default VS Code theme renders constants with a slightly different shade of blue.

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