Fix export_to_brat when there are spaces before new lines

[TheooJ]

Description

export_to_brat has been modified to allow the conversion of annotations detected on multiple lines, when there are spaces before newlines.

Ex with the input text:

[text] diverticulite \n
sigmoïdienne compliquée [text]

Before the change the output of doc2brat would have been:

T1	DISO 1732 1746;1747 1770	diverticulite  sigmoïdienne compliquée

Which is not readable by brat because 1) the extra space before the newline is not accounted for, I remove it for counting spans and 2) the begin and end spans of the fragment on the newline were not incremented.

After the change it becomes:

T1	DISO 1732 1745;1747 1770	diverticulite sigmoïdienne compliquée

Checklist

• If this PR is a bug fix, the bug is documented in the test suite.
• Changes were documented in the changelog (pending section).
• If necessary, changes were made to the documentation (eg new pipeline).

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b3c7ddd) 94.30% compared to head (24d3cc1) 94.31%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   94.30%   94.31%           
=======================================
  Files         246      246           
  Lines        6306     6293   -13     
=======================================
- Hits         5947     5935   -12     
+ Misses        359      358    -1     

Files Changed	Coverage Δ
edsnlp/pipelines/ner/umls/patterns.py	100.00% <ø> (+1.92%)	⬆️
edsnlp/connectors/brat.py	90.95% <100.00%> (-0.17%)	⬇️

... and 2 files with indirect coverage changes

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