fix parser: use context extend from nodes.util

[SPLYASHKA]

Replace local context_extend with nodes.util.snippet_extend_context

The default extend behavior was inconsistent between the functions (ls.snippet and ls.parser.parse_snippet / ls.parser.parse_snipmate)

Previously, when passing a string as the first argument (context), the parser ignored the 'extend' parameters (while ls.snippet not). The local context_extend function did not merge extend fields if context was a string. Now, using nodes.util.snippet_extend_context ensures that extend parameters are correctly merged regardless of whether context is a string or a table.

[SPLYASHKA]

For example (before changes)

local s_mathonly = ls.extend_decorator.apply(s, {}, { condition = tex.math_mode })
local s_mathonlyy = ls.extend_decorator.apply(s, { condition = tex.math_mode })
local parse_mathonly = ls.extend_decorator.apply(parse, { condition = tex.math_mode })
local math_auto = {
}

local greek = {
  parse_mathonly({ trig = "@a" }, "\\alpha"),
  parse_mathonly("@h", "\\alpha"),
  s_mathonly("@g", t("\\gamma")),
  s_mathonlyy("@u", t("\\gamma")),
}
vim.list_extend(math_auto, greek)

return {}, vim.list_extend(text_auto, math_auto)

s_mathonly, s_mathonlyy work perfectly fine, while parse_mathonly use condition = tex.math_mode only if you pass context as table.
("@A","@g","@U" - autosnippets with condition, "@h" - autosnippet without condition)