[LaTeX] Add support for \gls and \glspl commands

**What problem does this solve?**
These lines don't raise any error while they should
```
The \gls{prb} are doing it.
The \glspl{prb} are doing it.
```
This line however, raise en error about having spaces before a comma
```
A single downlink cell serving a set of \glspl{ue},
```
To my knowledge, the LaTex parser right now doesn't parse the \gls and \glspl commands, and simply return an empty string.

**Proposed Solution**
Implement a way so that \gls and \glspl are correctly parsed, by looking in the glossaries file defined by the \input command

**Component**
- [x] Core engine
- [ ] Plugin/Extension
- [ ] Other: _____

Start by tracing the LaTeX parser's handling of \gls and \glspl and how the \input command locates glossary files. Use the examples in the issue to verify that glossary terms are resolved and that the resulting text is checked correctly, including the comma case.

Written by the indexing model from the issue text.