fix(tostring): add footnote pattern to LPeg parser for correct table column widths

[ImmanuelHaffner]

Problem

The tostring module (lua/markview/renderers/markdown/tostring.lua) is used to compute the visual (post-conceal) width of table cells for column alignment. It had no pattern for footnote references like [^1] or [^label].

These fell through to the hyperlink_no_src pattern, which strips [ and ] but keeps the ^ character, producing a visual string one character too wide.

Input	tostring returned	Actual renderer shows	Width error
[^1]	󰌷 ^1 (width 4)	󰯓 1 (width 3)	+1
[^long-descriptive-footnote-name]	󰌷 ^long-descriptive-footnote-name (width 37)	󰯓 long-descriptive-footnote-name (width 36)	+1

This caused misaligned columns in any table row containing footnote references.

Fix

Add a dedicated footnote LPeg pattern and capture function that:

• Matches [^label] syntax, placed before hyperlink in the token priority list
• Strips [^ and ] delimiters, matching the actual renderer behavior (markdown_inline.lua conceals 2 bytes [^ and 1 byte ])
• Looks up the footnote config for icon/corner/padding decorations
• Returns the correct visual string for width measurement

Verification

✅ [^1]              → width=3  (was 4)
✅ [^abc]            → width=5  (was 6)
✅ [^long-label]     → width=49 (was 50)
✅ [link](url)       → unchanged
✅ [text]            → unchanged
✅ ![img](url)       → unchanged