[Feature] Create len builtin

[Ph0enixKM]

Is your feature request related to a problem? Please describe.
There was a problem with len function - where to put it, because it fits well in std/text and std/array. The len function is so atomic that should be implemented as a built-in.

Describe the solution you'd like

• Create a new module len
• Add len support for tree-sitter tree-sitter-amber#5
• Add len support for Vim amber-vim#3
• Add len support for Zed amber-zed#6
• Add len support for VSC amber-vsc#6

Describe alternatives you've considered
Moving len to std/core. This would complicate things even more.

Additional context
N/A