Fix DeclIds

[LoicRiegel]

Description of changes

What I was trying to improve

• decl ids used for keywords like "let", "if" etc... were defined statically, whereas they are generated dynamically in nushell. So the hard coded ids were very likely to become invalid every time we update nu.
• Some of the hard coded ids currently used were actually already wrong!

After my changes:

• The code now gets the decl names by id without the need for hard coded values
• Thus we use the right ones

The tests still pass.

Friendly reminder that's I'm still quite new to Rust, but eager to improve, so any improvement suggestion is more than welcome :)

Relevant Issues

None

[fdncred]

Thanks