Code is just moved from the the main completion function. No functional changes

Function is async and has a Cancellation token, though the later could likely be better made use of.

I've  duplicated the lineTextBefore variable within the function rather than passing it in. This is less efficient just now but I think it will be cleaner once all completion functions are separated out. Eventually I don't think it will be needed within the function at all.

code is just moved from main completion function.

No functional changes.

Just moved code from main completion function.

No functional changes.
lineAfterTest variable also moved in to this function as it is the only place it is used.

Code just moved from main function

No functional changes.

No functional changes, just tidying code.

required renaming of previous `mathCompletion` variable to `mathCompletionItems`

Just moving code from main to new function. No functional change.

Consider moving `mathCompletionItems` into new function in future

Naming needs further discussion.

Matches is type string array,

The line `matches = lineTextBefore.match(/\\+$/);` seems to be old code and is not doing anything of use. It is repeated a few lines further down in the if statement for maths completion.

Using named key-value pair for code clarity.

Previous capture relied on counting number of backslashes after matching a regex and performing modular arithmetic to check for an odd number.

This commit is to change the latexCmd regex to a single line which says:
`/(^|[^\\])([\\]{2})*\\$/`

```
IF ( ( (on new line) OR (following a non-backslash char) )
   AND (0 or more pairs of backslashes)
   AND a single backslash )
THEN there are an odd number of backslashes.
```

The benefit of this is to better align with the other regex's used in the if-else statement. also removes need for `matches` variable.