Major refactor that should dramatically improve performance by caching results of parsing files in workspace

Major refactor that should dramatically improve performance by caching results of parsing files in workspace for [[wiki-links]] and #tags (instead of re-parsing just-in-time, every time we want suggestions.)

Enhancements:

• cache results of parsing files in workspace. Closes #31
• support for mdx and fountain files and new config vscodeMarkdownNotes.defaultFileExtension allows you to set the extension that gets appended upon note creation. Closes #44 /ht @andermerwed

Fixes:

• newly created tags will be registered for auto-completion (once they file they are in is saved to disk). Closes #36 /ht @b3u
• line numbers are no longer off by one. Closes #35 /ht @b3u

Cleanup:

• remove unused RemarkParser stuff ( left in branch https://github.com/kortina/vscode-markdown-notes/tree/ak-remark-parser )
• renames for clarity:
  • ReferenceSearch to NoteParser
  • ContextWord to Ref
• cleanup / dry-up errant references to file extension match regexes, now at _rxFileExtensions