Enable std/oids and std/endians for JS backend

[Copilot]

import std/oids failed with JS backend due to std/endians calling copyMem, which doesn't exist in JS.

Changes

lib/pure/endians.nim

• Added JS-specific implementations using emit blocks for byte manipulation
• Handles three JS pointer calling conventions:
  • addr(array) → (Uint8Array)
  • addr(variable) → (variable, 0)
  • addr(obj.field) → (obj, "field")
• Object fields copy values directly (byte-level manipulation not feasible for JS scalars)

tests/js/tstdlib_imports.nim

• Enabled oids and endians imports

Example

import std/oids

let oid = genOid()
echo $oid  # Now works with JS backend

Previously this would fail at compile time with Error: undeclared identifier: 'copyMem'.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ambiguous error when importing lib std/oids with js backend</issue_title>
<issue_description>### Description

Hey, sometimes I accidentally import some modules that do not work with js backend, but the error I get is rather confusing, I spend a lot of time figuring out what piece of code causes this problem.

import std/oids

This is confusing because the module oids does not imported directly.

Nim Version

Nim Compiler Version 2.1.1 [Linux: amd64]
Compiled at 2023-08-13
Copyright (c) 2006-2023 by Andreas Rumpf

git hash: 9207d77

Current Output

/home/hamidb80/.choosenim/toolchains/nim-#devel/lib/pure/endians.nim(131, 57) Error: undeclared identifier: 'copyMem'
candidates (edit distance, scope distance); see '--spellSuggest': 
 (4, 4): '=copy'
 (4, 4): 'compiles'
 (4, 4): 'ctypes'

Expected Output

the above error + something that shows module import hierarchy

Foot Note

why std/oids should not work with JS??
</issue_description>

<agent_instructions>implements it for JS backend
test it with ./koch temp js -r test.nim</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ringabout PR is welcome.

• Fixes Ambiguous error when importing lib std/oids with js backend #22475

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