feat(notes): keep an extension typed with a new note's name

[dionmunk]

A new note always got the default extension, so query.sql became query.sql.md, starting with a # query.sql heading (#81).

Changes

• New Note keeps a typed extension when it's one a note can have: listed in notes.notesExtensions, or, with the default *, one VS Code has a language for. That list comes from every installed extension's language contributions, plus plain text (.txt), which VS Code's core registers itself rather than an extension. The .07 of Meeting 2026.10.07 isn't a language, so that name stays whole and gets the default extension.
• Only Markdown notes start with a heading, and it leaves out the extension (# plan for plan.md). A heading would make a new .json or .sql file invalid. People whose default extension is txt now get an empty file instead of a # Title line.
• Rename uses the same rule, which fixes two bugs with the default *:
  • renaming a.md to a.txt failed with "'a.md' already exists", because * was compared as if it were an extension and the typed one was swapped back
  • renaming to a name with dots lost its last part: Meeting 2026.10.07 became Meeting 2026.10.md
  • a note without an extension (README) had its whole name treated as its extension; it now renames as typed

Testing

• npm test: 29 passing, 3 new tests for splitting a typed name.

• End to end in VS Code 1.140 on macOS:

  You type	master	this branch
  New Note query.sql	query.sql.md with # query.sql	query.sql, empty
  New Note data.json	data.json.md with # data.json	data.json, empty
  New Note list.txt	list.txt.md	list.txt, empty
  New Note plan.md	plan.md.md	plan.md with # plan
  New Note Meeting 2026.10.07	Meeting 2026.10.07.md	same
  Rename a.md to a.txt	"'a.md' already exists"	a.txt
  Rename to Meeting 2026.10.08	Meeting 2026.10.md	Meeting 2026.10.08.md
  Rename README to README2	not checked	README2
  notesExtensions: md,txt, New Note query.sql	not checked	query.sql.md, so it shows in the tree
  default extension txt, New Note todo	not checked	todo.txt, empty

  The master column for query.sql, data.json, a.txt and the dotted rename comes from the same scenarios run against master; list.txt and plan.md follow from the same code path.

• New Note in both sections, the "where should it go" question, and New Note/New Folder with a note selected were re-run with the same results.

Closes #81.