Close mozilla-mobile#12623: Do not match issue ID if preceded by othe…

…r characters

To avoid matching against issues in other repositories, we can reduce
our matches to not include values if they are preceded by 1 or more
string/number/hypen/underscore values.

Tested using:
 - a JS console:
 ```javascript
 const issueRegExp = new RegExp("(?<![A-Za-z\\-\\_]+)#(\\d+)+", "g");
 const numMatches = Array.from(issueRegExp[Symbol.matchAll]("mozilla-mobile#12345")).length
 const numMatches2 = Array.from(issueRegExp[Symbol.matchAll](" mozilla-mobile#12345 ")).length

 assert(numMatches == 0);
 assert(numMatches2 == 1);
 ```
 - regex tester: https://regexr.com/6rg2v

.github/workflows/issue-linker.yml

@@ -17,5 +17,5 @@ jobs:
         uses: gabrielluong/[email protected]
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
-          commit-regexp: "#(\\d+)+"
+          commit-regexp: "(?<![A-Za-z\\-\\_]+)#(\\d+)+"
           section: "### GitHub Automation"