feat: edits to mission source for the editor's forms and map

[vdmkenny]

The editor's record forms and Scene view (OpenReliant/openreliant-editor#8, OpenReliant/openreliant-editor#2) change records in the source as they're edited. This adds what they need from the library:

• Where each node ends. The bridge copies the end of each node from Luau's parser. A table's item runs from its key to the end of its value.
• Edits (dte.source.edit): set an argument, set or remove a field of a declaration's table, and add a declaration. Each edit replaces one span, so comments and layout stay. A new field goes on a new line with the same indent in a table over several lines, and on the same line otherwise; a trigger without fields gets a table before its function. Values are written as the decompiler writes them.
• language.fieldsOf: the table of fields each declaration takes and which argument it is, for the editor's forms. A test ties it to the declarations' signatures.
• events.reached_reach is public, for the map's ship_reached range.

Checks: zig build test (2665/2666, 1 skipped), zig fmt --check, make check-files, make check-missions (75 missions, 0 failing; the guide's change builds).

Closes #1210

[vdmkenny]

Closing. The editor makes its own edits to the source, so OpenReliant's library stays as it is, and the link stays the original's protocol plus the messages that docs/engine/editor-link.md and #1126 document.