Custom words not recognized when italicized in markdown

[ms609]

Description

Words in a custom dictionary are underlined in red and treated as misspelt when surrounded by _italic_ markdown

Steps to Reproduce

1. Add a custom word to the "Known words" list in the spell-check settings page: e.g. FooBar
2. Type the word in a Markdown document and verify it is marked as correctly spelt
3. Italicize the word by typing _FooBar_. The word is marked as incorrect

Expected behavior:
Formatting marks should not influence the correctness of the word

Actual behavior:
Underscores (but not asterisks) are treated as part of the word, causing the word to fail to match the custom words list

Reproduces how often:

Always

Versions

Windows 10

Atom    : 1.60.0
Electron: 9.4.4
Chrome  : 83.0.4103.122
Node    : 12.14.1

apm  2.6.2
npm  6.14.13
node 12.14.1 x64
atom 1.60.0
python 3.9.7
git
visual studio

Additional Information