fix: add support for underscore italic/bold, subscript, and superscript

[ericclemmons]

Summary

This PR adds missing markdown stripping patterns for width calculations in concealment mode:

• _italic_ and __bold__ (underscore variants)
• ___bold+italic___ (underscore variant)
• ~subscript~ (single tilde)
• ^superscript^ (caret)

Problem

When using markdown like **_Bold Italic_** or Sub~script~, the underscore and single-tilde patterns were not being stripped during width calculation. This caused misaligned table columns because the concealed characters were still being counted toward the display width.

Example

Before this fix, a table with these cells would be misaligned:

| Style | Example |
|-------|---------|
| **_Bold Italic_** | combined |
| Sub~script~ | H~2~O |

Changes

Added 5 new regex patterns to getStringWidth() in the markdown stripping loop:

.replace(/___(.+?)___/g, "$1") // ___bold+italic___ -> text
.replace(/__(.+?)__/g, "$1")   // __bold__ -> bold
.replace(/_(.+?)_/g, "$1")     // _italic_ -> italic
.replace(/~(.+?)~/g, "$1")     // ~subscript~ -> subscript
.replace(/\^(.+?)\^/g, "$1")   // ^superscript^ -> superscript