standardize_variables: Use lookup table of parameters rather than text-matching

Need to construct a table matching parameters in `rems::ems_paramaters` to those in `wqbc::limits`

| limit_name | ems_parameter | ems_param_code |
| ----------- | ---------------- | ----------------- |
| X | X Total | 123 |
| X | X Dissolved | 1234 |

Then `standardize_variables` should lookup the limits in `wqbc::limits` using this table, then output from `calc_limits` should look like this:

| Variable | Value | LimitName | UpperLimit | LowerLimit |
| -------- | ------ | ----------- | ----------- | ----------- |
| X Total | 7 | X | 11 | 2 |
| X Dissolved | 6 | X | 11 | 2 |
| X Suspended | 5 | X | 11 | 2 |
| X Foo | 42 | X | NA | NA |

`X Foo` was not matched in the lookup table

Then `calc_limits` should have an argument `drop` saying whether or not to drop the rows where limits were not able to be calculated - either because it wasn't able to be matched in the lookup table, or the limit was dependent on another variable that was not available.

Start by reading the existing standardize_variables and calc_limits entry points, along with rems::ems_paramaters and wqbc::limits. Define the parameter lookup mapping and verify that calc_limits returns matched limits while the drop argument controls unmatched or dependency-missing rows.

Written by the indexing model from the issue text.