feat: add definition of pushdown automata

[shetzl]

Add the definition of pushdown automata and their two acceptance conditions: acceptance based on empty stack and acceptance based on final state.

Co-authored-by: Tobias Leichtfried [email protected]

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[github-actions]

PR summary 90c0386753

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Computability.PDA (new file)	544

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Declarations diff

+ Conf
+ PDA
+ Reaches
+ Reaches₁
+ acceptsByEmptyStack
+ acceptsByFinalState

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).