Add description support for operations, fragments, variables and schemas

[hugo-vrijswijk]

The September 2025 GraphQL specification allows descriptions on every definition.

Adds description support on:

• operations
• fragment definitions
• variable definitions
• schema definitions (since September2025)

Also adds support for schema descriptions in the introspection system.

Six conformance test cases now pass.

Also rewrites the parser a little bit to only create Parser instances once, improving performance. Here is a very approximate benchmark:

document                  main      branch    change
query x1               38.1 us      4.8 us      7.9x
query x2               42.0 us      8.6 us      4.9x
query x4               47.8 us     15.4 us      3.1x
query x8               61.0 us     28.6 us      2.1x
query + fragment       35.9 us      5.1 us      7.0x
sdl, 7 defns          261.8 us     13.7 us     19.1x
schema defn            65.6 us      4.3 us     15.2x
introspection          82.7 us     50.3 us      1.6x

[milessabin]

LGTM!