Add support for (multi-line) footnotes

[shaksiper]

When the foot note is not a singular word [^word] (non-space), it behave as intended with working LSP actions such as rename, go to definition etc. However, when the foot note is a text with space, it will not work as a foot note, LSP actions, highlighting etc will not work for that foot note.

Example text: [^word]: I suspect that it is treated as a regular link

Testing paragraph [^test-note] with footer note [^test-note2]

[^test-note2]: Footer
[^test-note]: Footer note

Environment details:

❯ nvim -v
NVIM v0.10.0-dev-2185+gae3eed53d
Build type: RelWithDebInfo
LuaJIT 2.1.1706032906
Run "nvim -V1 -v" for more info

❯ marksman --version
1.0.0-f124c56+f124c5616cf38241e1a5e58808ed78e2d9a1d385