Refactors picker usage to decouple PlatformIO command logic from UI implementation.

[anurag3301]

This PR creates ability to add more picker as asked in #43 and #62

• Introduced a shared picker interface under lua/platformio/pickers/ with backend selection via picker_backend (auto | telescope | ui_select).
• Added backend modules:
  • pickers/telescope.lua
  • pickers/ui_select.lua
• Refactored :Pioinit to separate:
  • board/framework data + init flow logic
  • picker UI rendering
• Refactored :Piolib to separate:
  • API fetch + install flow logic
  • picker UI rendering
• Refactored :PioTermList:
  • moved command logic to lua/platformio/piotermlist.lua
  • routed terminal selection through picker abstraction
  • removed inline Telescope setup from command path
• Added help docs for picker config:
  • :help platformio-picker in doc/platformio.txt

Why

• Makes picker engine swappable without changing command logic.
• Enables adding new picker backends with the same API.
• Reduces coupling and side effects (especially around Telescope config in runtime commands).

Config

  require('platformio').setup({
    picker_backend = 'auto', -- auto | telescope | ui_select
  })

Notes

• auto prefers Telescope and falls back to vim.ui.select.
• Existing command behavior is preserved; only picker/UI plumbing was reorganized.