feat(stdlib): vim.json.encode(...,{escape_slash:boolean}) #30561

Problem:
vim.json.encode escapes every slash in string values (for example in
file paths), and is not optional. Use-case is for preventing HTML
injections (eg. injecting `</script>` closing tag); in the context of
Nvim this is rarely useful.

Solution:
- Add a `escape_slash` flag to `vim.json.encode`.
- Defaults to `false`. (This is a "breaking" change, but more like
  a bug fix.)

test/functional/lua/json_spec.lua

@@ -152,6 +152,45 @@ describe('vim.json.encode()', function()
     clear()
   end)
 
+  it('dumps strings with & without escaped slash', function()
+    -- With slash
+    eq('"Test\\/"', exec_lua([[return vim.json.encode('Test/', { escape_slash = true })]]))
+    eq(
+      'Test/',
+      exec_lua([[return vim.json.decode(vim.json.encode('Test/', { escape_slash = true }))]])
+    )
+
+    -- Without slash
+    eq('"Test/"', exec_lua([[return vim.json.encode('Test/')]]))
+    eq('"Test/"', exec_lua([[return vim.json.encode('Test/', {})]]))
+    eq('"Test/"', exec_lua([[return vim.json.encode('Test/', { _invalid = true })]]))
+    eq('"Test/"', exec_lua([[return vim.json.encode('Test/', { escape_slash = false })]]))
+    eq(
+      '"Test/"',
+      exec_lua([[return vim.json.encode('Test/', { _invalid = true, escape_slash = false })]])
+    )
+    eq(
+      'Test/',
+      exec_lua([[return vim.json.decode(vim.json.encode('Test/', { escape_slash = false }))]])
+    )
+
+    -- Checks for for global side-effects
+    eq(
+      '"Test/"',
+      exec_lua([[
+        vim.json.encode('Test/', { escape_slash = true })
+        return vim.json.encode('Test/')
+      ]])
+    )
+    eq(
+      '"Test\\/"',
+      exec_lua([[
+        vim.json.encode('Test/', { escape_slash = false })
+        return vim.json.encode('Test/', { escape_slash = true })
+      ]])
+    )
+  end)
+
   it('dumps strings', function()
     eq('"Test"', exec_lua([[return vim.json.encode('Test')]]))
     eq('""', exec_lua([[return vim.json.encode('')]]))