Fixes#15248
Add typescript to dictionaries in cspell.json
Tested by adding `instanceof` in any .md file and run `npx cspell --config .vscode/cspell.json`
```
npx cspell --config .vscode/cspell.json README.md
1/1 .\README.md 495.98ms X
c:\Users\User\Desktop\test\azure-sdk-for-js\README.md:55:110 - Unknown word (MSRC)
c:\Users\User\Desktop\test\azure-sdk-for-js\README.md:55:325 - Unknown word (MSRC)
CSpell: Files checked: 1, Issues found: 2 in 1 files
```
a2aebbe097