cql2 does not accept single character attribute names

Right now, single character attributes cannot be used in CQL2 filters:

```python
❯ pygeofilter parse cql2_text "a = 1"
Error: No terminal matches 'a' in the current parser context, at line 1 col 1

a = 1
^
Expected one of:
* INT
* S_CROSSES
* NOT
* MINUS
* S_OVERLAPS
* SINGLE_QUOTED
* S_TOUCHES
* S_INTERSECTS
* INCLUDE
* DOUBLE_QUOTED
* S_CONTAINS
* BOOLEAN
* EXCLUDE
* LPAR
* FLOAT
* S_EQUALS
* S_DISJOINT
* S_WITHIN
* ENVELOPE
* /[a-zA-Z][a-zA-Z_:0-9.]+/
```

The challenge is that final regex which doesn't match single character names:
https://github.com/geopython/pygeofilter/blob/main/pygeofilter/parsers/cql2_text/grammar.lark#L143

Start with the attribute-name terminal regex at line 143 of pygeofilter/parsers/cql2_text/grammar.lark and reproduce the issue using the provided pygeofilter parse cql2_text command. Adjust the grammar so a single-character attribute parses, then verify that both the reported case and existing multi-character attribute names continue to work.

Written by the indexing model from the issue text.